I know the X-Men are prominent players in the Marvel Universe, and that many people believe that Disney can save them after Dark Phoenix. But adding them to the MCU would potentially ruin the whole universe, and here's why.
First off, all X-Men are mutants, and a key factor in the X-Men, is that they are feared and hated by ordinary humans, and mutants have been around since at least the 1800's. And people have known about mutants for a LOOOONG time in universe. How can audiences buy ordinary people living in the MCU accepting the non-mutant heroes, yet fearing and shunning mutants, after everything the Avengers, Guardians, and TV characters have done for them?
Also, if mutants and/or X-Men have been active, people in and out of universe are going to be asking a big question that I doubt anyone can answer using in-universe logic. "WHERE WERE YOU!?" Loki, Ultron, Zemo, and Thanos were threats to all living things, and in the comics, whenever these villains threaten at least the whole world, the Avengers are often joined by the X-Men, and they fight together. Also, how is it that no one is aware of the X-Men in the movie universe? In the comics, the X-Men are a household team alongside the Avengers, EVERYONE knows who they are. A MCU X-Men team operating in secret is too much of a suspension of disbelief and raises too may questions.
And if you try to avoid all of that, and make it so that in their first movie, mutant powers reveal themselves, even THAT will backfire. Because as I've mentioned before, people have known about the other MCU heroes, and for the most part, accept that they have powers beyond mortal men. Having them all of a sudden fear individuals who have powers, when there are already individuals with powers, is too much suspension of disbelief. And also, the MCU has tweaked a number comic elements, IE having the Tesseract, Either, Loki's Staff, Eye of Agamotto, etc, be Infinity Stones, when in the comics, they are their own items. Tweaking the nature of mutants so that let's say, they are a side effect of something that's already happened in the MCU, defeats the purpose of mutants. The mutation gene that gives mutants their power, is a natural, biological element that certain people can have, like being on the autism spectrum.
In hindsight, I think the MCU is too heavily established to bring in mutants and the X-Men without extensive retcons, and breaking the limit of the suspension of disbelief.
