Well, Carol Marcus wasn't entirely new technically, even though she didn't feel like the same person,even allowing for the AU, as in STII. She didn't win over fans but it was on the basis of her actual character not a minority status. I don't see Trek getting rid of Sulu due to homophobic backlash. I'm sure there will be some ugly internet comments, there always is, but I still don't see it being that big a problem. Sulu will get a subplot, but it's not going to be Gay Husbands in Space. We don't even know if an actor is being cast to play Sulu's husband. He'll probably just be mentioned in passing. It may well be a blink and miss it moment, such as Sulu mentioning that he misses his family in a one minute character moment.You mean one that no one has any connection to, that they can easily dispatch when the fan base gets riled up? Like Carol Marcus?
I think it would be a huge betrayal of Trek's values if the concept of the closet still existed in 23 rd century Earth when humanity should have grown beyond homophobia along with racism and sexism.