I just think the implied original premise was lacking. If it does go a forced NTR path, Why would bro get isekai'd and want to stick around while being cucked? Find a different demon/etc waifu to be exclusively drained by at that point imo, because that would be a bit underwhelming/another "I've seen this 50 times before".
If there's branching paths where the MC can get with other waifus you can roll with all sorts of stuff. Then you can have that sort of route with them if that's the dev's thing, with various other content (yandere, dom, more wholesome ones, maybe even one who wants to be dommed by a human for comedic effect, bigger waifus like an oni-woman or something, etc) with others. Win-win, its ai, so dev can whip up cgs for whatever. This is supposed to be another realm with different sorts of beings after all. Limiting the plot when giving the player agency would be better than an uninspired plot
or an illogically forced NTR. Adding a variety of a few more waifus for variety would be good. Then people who don't like it can ditch them as soon as able where others can sit in the corner chair playing it on joiplay.
That powerful beings won't want to take the strange/unique/rare otherworlder for themselves, especially if he's not valued by a mere succubus? Nah they'll cuck the succubus instead and steal him if anything.
(Inb4 the "NTR" was going to be "reverse" NTR all along with that sort of thing and people spooked off the dev
).