I think the OT fans were already passed the acceptance stage about the end of this franchise a long time ago. If you've been following these boards, the once-ardent fans of these movies, the OGs who've been posting on these boards since it was Spider-man Hype or even the ones that were posting during X3 days were very much against most of the things that were announced for this movie, not the least of which was Kinberg's involvement.
We all (well most of us did) saw the writings on the wall. We all (w.m.o.u.d.) knew this was going to be a cluster****. The OT fans had to move on because the studio left us no choice. Of course there was a small minority of us that actually enjoyed Apocalypse the first time we saw it (hey...my theatre clapped at the end of the movie!) but have since seen the error in that.

But for the most part the very loud, majority of the forum knew this movie in particular wasn't going to be good and was ready for whatever Feige has cooked up.
The only people who were completely (?) blindsided by how bad Dark Phoenix turned out are:
(1) Sophie Turner fans who were excited at the prospect of Sansa as a lead in a film (I'm not sure what will come of her career from this train wreck);
(2) people who believed that because a writer/director/producer apologized for a prior film that gives them the magical skills to make a better product;
(2.1) Kinberg only fanboys (I didn't know they existed until this movie) "I gave Singer a chance...after X-men and X2 and DOFP...and he gave me Apocalypse...now I'm giving Kinberg a chance." That's assuming it's as easy to make a good movie as it is to make a bad movie...
(3) McAvoy and Fassbender fans who think those two actors in particular can polish a turd;
(4) fans who only listened to insiders who liked their screening and turned a blind eye to every other negative intel;
(5) fans who will love any X-men product no matter what (admittedly I used to think I was one of these people. Dark Phoenix was the first movie from this franchise that I didn't see in theatres...don't ask me how I saw it
