The problem is you'd have to rewrite the entire X-Men and by extension, mutants, history.
And I don't mean slightly. You'd have to change damn near every character's entire back ground.
Fans were already pissed off MCU Wanda and Quicksilver had no ties to Magneto. You're essentially asking them to do that to
every character.
Yeah but the difference is this is shown in the very first movie in the X-Men franchise.
The fact that the 2000 X-Men movie starts with a Senate meeting on mutants establishes that this is well known in this franchise, despite it being the 1st movie.
It would only be similar if something like this was mentioned in "Ironman" in 2008. Even in passing.
As it stands we're damn near 30 movies deep in this cinematic universe
without a scene like that to establish that mutants had been around.
And truth be told the Fox X-Men time line is absolute gibberish so they never committed to a definitive origin, anyway. We went from "mutants are a recent development in 2000" to "mutants damn near started WW3 in the 1960's".
Fred.