Exactly. I think it is preferable for mutants to have always existed, but just in very small numbers, and for their existence to have been kept secret. Just like it had been secret up until a short time before the first X-Men movie and just like it had been secret in First Class until they revealed themselves to the world in that film. As you said, just basically treat it in the MCU similar to how they did in the first X-Men film. In that movie the mutant phenomenon was something of which the general population was just beginning to become aware. The MCU can say that their numbers were very few and those who developed abilities hid them, or were hidden by others, from the world. Those that physically looked different would just be though of as “freaks”. After the use of the Infinity Gauntlet on Earth the number of mutants begin to increase. During the 5 years between Infinity War and Endgame the mutant population continued to expand and efforts to keep their existence a secret have become more difficult due to the increase in numbers. And that would bring us into “modern day” 2023/2024 when their existence is revealed. That’s the way I’d go with it. It’s simple and clean and similar to what has already been done in the previous franchise where it was established that Mutants existed for a long time secretly before their existence was revealed to the general public. And it leverages a major event in MCU history as a catalyst for the sudden increase in their numbers. In other incarnations (comics and/or films), hadn’t it been stated or speculated that the “atomic age” was thought to have been a reason for the increase of the mutant population in the 20th century?