I will disagree on a couple of your points.
First, even if you call it "retconned incest", I would argue that many of these games are written with family relations in mind and then, correctly called out by you, tamed down so certain ways of income can be supported. Which might leave them in a limbo - you talk about how some interactions do not feel like family, only the general set up is there, while I would argue that certain set ups do not really make sense with the "unpatched" version, e. g. MotH (which you also don't argue to be an example) or MWM (which you do argue, but I would say that the original relationships do not make sense). Either way I would argue that many interpersonal relationships in some games do not represent how non-family would treat each other. It will of course always depend on the game. And as you say, also for me the kink isn't satisfied if the taboo isn't there, I played an (otherwise enjoyable) game where MC had a relationship with his sister - undoubtedly sister - but nobody really cared because society has moved on or suchlike. Doesn't work, regardless of the "family depiction".
However I will also disagree that this is done to "draw suckers in". I would guess the most common situation is not somebody has a game and wants to make more money, so slap some incest relations haphazardly onto the game, but a game that is toned down as much as necessary to make avenues of payment possible - it's either not making the incest solution unavoidable or not having enough money to make the game at all. And not every dev is cut out to be one of your "heroes" to use uncommon alternatives and risk tons of money. Calling those who just want to make a game without having to fight the system, and then do what they can within these boundaries "patreon whores" is pretty strong and in my opinion also pretty wrong.
As for the tag system - which is reworked because it has many flaws but I doubt this one can really be addressed - it is not exclusively to find games with your kinks, but also to be able to avoid kinks which disgust you. And as much as some random people just with a moniker "sister and brother" do not satisfy those looking for incest, just calling them "landlady and tenant" will not put those people at ease when it is obvious that they are not. Same reason why some kinks are tagged even though they only happen once, in the background, without involving the MC - those who want this kink won't enjoy the game, but those who despise it can steer clear.
Like I've said before, the problem (at least to me) is that so many people confuse facts with opinions.
They talk about things they heard other people saying or only know half of the facts about, and then fabricate these theories that they post and other like-minded people use to base their "facts" on. And so the circle continues.
Most devs probably don't feel like addressing or correcting these posts either, as the posters usually don't really want to engage in an actual discussion, but they just want to bad-mouth the dev to vent their frustration.
I assume because the game doesn't have exactly the content they want?
I have to say, this puzzles me though. If a game doesn't offer what you want, that's fine, but why make a big deal out of it?
Just go play a different game, there's plenty on offer (and it's not like you are paying for it...).
As for incest (or any fetish), I don't think that it's necessarily that black and white:
Yes, if you make a game focused around one fetish, it might be a different story (arguably).
But many of these games tend to have multiple fetishes included (as most non-niche porn does).
Why? To appease as many people as possible, because contrary to popular belief, a dev has bills to pay just like most "real people". To me, it's like saying a game only has pregnancy content if you can fuck a pregnant woman in her ass.
Just that a specific fetish or topic is your kink, doesn't mean that is the definition of the fetish.
But that doesn't mean that content is slapped on. Or it is too "draw suckers in".
(What a small way of thinking: everyone who likes something I don't like is a sucker. Good luck with that!)
Then there's also their own creative freedom and preference (while tiptoeing around the rules of certain platforms).
I, for one, create what I enjoy creating and as a dev have the luxury to do that (and feel the consequences of it).
If I feel a certain fetish/scene has a place in my story, I will include it.
I know all too well it won't please everyone. Some will say it's not enough, others will say it's too much, etc.
But that's just the way it is.
Lastly, there is the vocal minority. In most cases (in general, perhaps) there is a small group of people who complain very loudly vs a much larger group that tends to just enjoy something or ignore it. I even see this on my own Patreon polls at times (and those are even biased in my favor).
So yeah, I guess I get why some people get frustrated because they like very specific content and it's not included in a game.
I get comments and messages about this on a daily basis.
"Why isn't X in, why doesn't Y have tattoos, why can't I rub apricot ice cream on my balls and have a nun throw darts at them?"
I think some people just fail to understand that even though it's great when you find a game that ticks all the boxes for you, not all (if any) games are made specifically for you.
Sure, that can be disappointing, but isn't that just how the world works (or used to work)?