Discussions in the MCU wiki discord got me thinking. IF... and that's only an IF, Secret Wars creates a whole new main MCU reality where the Fantastic Four and X-Men always existed... then that actually means that canoners won and there's absolutely nothing that can be done to disprove them.
Think about it... if the history is even slightly different at all, even if it appears to be pretty much the same for the movie characters, this means that they could introduce new versions of the Runaways, of the Inhuman Royal Family, of Quake, of Cloak and Dagger, and it wouldn't actually affect the canonicity of anything. The MCU versions are still canon, they just existed in the pre-Secret Wars Earth-199999, one without (speculative) reality warping to place certain characters and teams together.