Adding an NTR ending would ruin the game for me retroactively, as I would imagine it would for most netorase fans. Nts and ntr fetishes are very far removed from each over. The main attraction of netorase content for me is experiencing this fetish in a safe fantasy-like environment, where it doesn't lead to real world consequences like cheating, betrayals, breakups, etc. Adding the aforementioned consequences as an option would destroy this fantasy. It would be the absolute turn-off.
That is why netorase fans fight tooth and nail to protect their favourite games from NTR cultists.
I'd like to add on to this while everyone has different tastes, I hard agree that nothing kills a prospective NTS game more than adding NTR. TBH, I'm not entirely sure WHY people like NTR, as I despise it. Not trying to kink-shame here or anything, but I honestly don't understand the appeal, so I can only make guesses at what makes it enjoyable. Are they putting themselves in the 'bulls' position? I don't really know. I've heard that NTR is about the emotions it causes the reader... which bluntly don't make me horny lol. Just not my kink.
For me at least, with NTS, there is literally nothing hotter than having a loving wife/GF go out and have extremely satisfying sex (with or without your participation - I do think swinging is hot, but that's slightly different topic), and then COME BACK to the MC and still be in love with him.
Call it self-satisfaction if you will, but the idea that 'my' lover is sexually liberated, knows how to enjoy pleasure, and still comes back to me? That's peak. So yes, fantasy (my life is NOT that good lmao), but it's why I love NTS as a genre as a whole. As I said, NTS *is* my kink! Just a shame that a lot of NTS has to have the added emotional twist of NTR. Honestly, light 'what if' that end up not being NTR DO add to the spice! It makes the story better. But if it ends in NTR then i just feel like shit and I wasted my time.
It's the suffering of a NTS enjoyer who's ALSO a completionist: I have to watch every ending of every game/VN i play, so games that have utterly fantastic NTS routes can sometimes be ruined by *that one NTR ending route*. As I said, it's a rough life being a completionist NTS fan... (lol)
But yeah, those are just my specific tastes, but if you peruse the various Master Netorase threads, you'll find that many NTS fans also hate NTR.