The main issue with a 40yo MCs is that he's precisely 40yo. Not that it's an old age, but this imply that he have ~20 years of life habits behind him. During those years, he shaped his life for it to be the most comfortable and pleasing possible.
If there's such thing as the mid-life crisis, it's because there's people who realize that this comfortable life is in fact not pleasing them. And it generally happen around 40yo, because it's, globally, the moment when you achieve to build it.
The older you get, the less receptive you are to changes. You worked hard to shape life to your measure, and don't feel like you need to do it again unless you know for sure that it will lead to something better.
It's something that don't apply to a younger MC, because the changes are mandatory. You can't continue to live as a child when you enter college. You can't continue to live as a restless teenager when you become an adult that now have obligations due to his job. Those are moments in your life when it's mandatory for you to change your life style, and your sexuality is part of this life style.
As writer you don't need to say more. Everyone have lived through those moments, everyone know that it imply a life style change. And it's mandatory steps, everyone will pass through them.
But for a 40yo MC, there's not many moment like this. Globally speaking there's three:
1) When your last child leave the house. Your life was shaped to include them, now you've some wall to breaks and others to build.
2) When the last of your two parents die. Whatever how adult you were, until that moment you were still a child, their child. Whatever your age, they were there when you needed for an advice. Now, you're the head of the family, and it change many thing in your mind.
3) When you retire. It's a radical change. Half the life you shaped was "work", and you now have to fill that half with something else.
Yet, the third apply to 60yo MCs, what is a bit too old for this kind of games. Except tragedy, the second apply around the same age, what only leave the first as possibility.
What doesn't mean that there's no other options, but those options will need to introduce a new perspective in MC's life; to open a window to the guaranty of an even more pleasant life. This simply because no one will want to leave the comfortable and pleasant way he live, especially after having past half his life to build it, unless he know that it will be for something more comfortable and more pleasant.
[forenote: My comments apply to "it's the only factor". Combination of factors obviously imply a different reaction]
A divorce, being a widow, those are hard times, but it's precisely to be able to life through hard times that you shaped your life "that way".
If the ex haven't been a bitch during the divorce, or wasn't a serial cheater, MC will just dive in his comfort and wait some times before he search for someone else. Same for a widow, but with a longer period before he starts to search for someone.
But if she have been a bitch, he can want to change, to prove her wrong; "So, I'm a boring idiot who don't know how to please a woman? Well, let's see what you'll say when I'll be surrounded by my harem of teenage girls."
Same if she's was a serial cheater. All those years wasted being faithful to a woman ready to suck a dick for a coffee...
2. A new and unusual woman moves into his life and he's instantly hooked.
If he's hooked, he'll not care about the other girls.
It's obviously not impossible to make a story with this, they don't all need to be harem. But I'm not sure that there's really a game that have one and only one possible LI. It would be a pure VN telling the story of "how I met my [whatever number] wife".
At most you can make it a moral choice for the player, his wife, or the new girl. It double the number of girls in the story, but you have to be careful because players will try to have them both, what must be impossible and lead to MC having none.
Now, you can also have this girl be the one that open the window I talked about.
Like for female MC, someone enter his life and he discover how sexually frustrated he his. But for this, the new girl need to be the one that will hit on him. He'll be hooked because she did everything for this.
This make MC relatively passive in the first half of the story, then actively pursuing girls in the second. A switch a bit too confusing for a game. It would be like a Mass Effect game where during the first half Shepard is a basic soldier going where (s)he's asked to go, then in the second half what (s)he's actually be in the games.
The only way to not suffer from this dichotomy is to goes the femdom route.
3. Like a younger guy, non-magical change of environment can happen. Being posted abroad (i.e. embassy, mission, intel work), or in a different gender balance setting (becomes a teacher at an all-female school). Crash landing in the Amazon jungle with a bunch of cutie-patooties. Inheriting a sex-related business from your shady uncle. Classic uncle move.
Just changing job will change nothing in his life. He'll have to do some adjustment, but that's all. He can't anymore workout at 7PM because his new office is too far, well, he'll do it at 8PM. He can't anymore have his Wednesday movie theater night because Thursday is a heavy day at works, well, he'll do it on Friday. And so on.
Moving to another place in the world would be a bigger change, but not that big because he'll carry his habit with him. The first thing he'll do is to unpack what make his life pleasant and comfortable. Let's say that he like to read, he'll build a bookshelf and unpack his books before he finish to unpack his clothes. It's what make life worth to be lived, and what will comfort him face to the mandatory change that just happened, so it's the priority in his mind.
One or the other implying an (almost) all girls surrounding don't change much to this. At least unless one of the girls starts to hit on him, what send back to the point above.
But a radical change, like a plane crash, yeah it can works. But only if it lead to a journey without possible return; or at least a situation that looks like it. MC will have to adapt to his new condition, mourning his past life and starting to build again something comfortable and pleasant with what he have access too.
But if there's a possibility for him to return back to his previous life, he'll not care about what surround him, he'll just want his comfort and pleasure back.
4. Mid-life crisis / change of morals.
As I said above, a mid-life crisis would works.
But a change of moral do not come from nowhere. You don't pass to average joe to a sex crazed just because you were fired or had to past through some trauma. It need to be something really big, like by example what Darthseduction
(hope he's healed now and have a great life) did with
The Things We Do For Love starts. The whole dynamic between the brother and sister, with the implied incestuous relationship that was planed, being due to the sister killing their father to save her brother's life. It bounded the sibbling and what was previously not even a thought in their mind became the center of their interest.
5. Answering a challenge with total domination.
With total submission, yes.
But if he had what is needed for a total, or even partial, domination, he would already, at least partly, live that way. One do not awakanen a morning, and discover that he have balls of steels. He can have partly refrained his impulse, but not totally.
Yet it can works, as long as it follow a slow pace. Enough is enough, and one day MC dared to talk back to his bullying boss/coworker/whatever. Terrified by what he did, that is so much "not him", but amazed by the positive result, he suddenly understand that he have no obligation to always comply.
But one night isn't enough to pass from full life of docility to a life where you decide what you'll do. It need months to achieve this.
6. And yeah, broadly speaking, magic. This one can be broken down into several full sub-categories.
It's the cheapest approach. Not by itself, but because it's generally used as an excuse; "shut up, it's magic!" But a well crafted story can perfectly works with this.
By example, in a medieval fantasy setting, MC can perfectly be someone important (mayor, prince, whatever) that is honest, loyal, and therefore an obstacle to anyone who isn't. He'll be (slowly or not) corrupted through magic, because a depraved pervert him woulld be too occupied fucking any cunt that come to close, to care about what his done in the shadows.
7. Finally, none of these. You don't need a man to be faithful to his wife for 20 years, or living in celibacy. You can have a man, or a woman, that simply goes on an adventure at the age of 40. Harrison Ford was something around 40 in Lost Ark, and realistically, the plot would work just as well with a 45 y.o. Indy.
The issue here is that adult games have the bad habit to have MC's sex life as main story. Therefore a 40yo MC that have an active sex life full of girls would be good to fap, but borring as hell because it would starts out of nowhere, then end while you know that there's still so much more to come.
The game would starts with something like: "It's monday, I'll go to works, then I'll pass the night with my monday fucktoy like I do every monday since years. Oh, wait, I need to remember to not overdo it, the twins are in town tomorrow, two years that I haven't seen them, I'll need all my strenght and probably don't sleep a second".
Then, one day when the dev would have enough, it will end with something like: "well, it was a good day. I'm sure that tomorrow will be good too, no reason for it to change until my death".
For it to works, the story need to focus on something else, totally external to MC's sex life. So, to keep your example, it would be the journey of Indy searching for the Lost Ark. A journey filled with sex.
There's games like that,
The DeLuca Family,
Heavy Five,
The Intoxicating Flavor,
Desert Stalker, by example. Yet the first twos don't have much sex
(and the second is not dead), while the last one is a weird hybrid. MC have always been a free spirit when it come to sex, and having a harem is natural in the game setting, but he only now starts to build his own one, despite the many opportunities he had in the past.
And, of course, among them, only two,
Heavy Five and
Desert Stalker, have a MC that isn't necessarily 40yo, but clearly isn't a twenty something one.
TL;DR:
As I said in the post you quote, it's not impossible, just more difficult.
Of course, one can use all the premisse he want and make a game based on this. Even "what a dream it was, it surely mean that I need to have a lot of sex with all the girls I met" could be used to start a story with a 40yo MC.
The difficulty isn't to find a reason for the MC to drown in sex, but to find one that is believable enough.
Yes, it is beneficial for a story to have some non-sexual plotline happening, and not just slice of life slop.
