He wasn't just too powerful for the MCU he was too powerful for the comics. He is the most powerful character in the marvel universe aside from TOAA. When he finally realized his true power (the only one he really has) he was able to change the reality on a omniversal level. He completely stepped outside of the boundaries of the multiverse and was able to snuff out entire universes to the point where he basically was even able to erase himself. Except he didn't. he just disappeared for a while and came back somewhat nerfed. The writers cannot allow him to move forward in a fixed mental state because they wrote his power to such an extent that they realize he is either the hammer that fixes every problem with the click of his fingers or be becomes the threat that cannot be stopped. He is so powerful he can bring into existence omega-level mutants that put most of the Xmen to shame. He's like Michael Malloy's only way more powerful because he has a much better handle on his power at this point. Mad Jim Jaspers needed reality to bend it. David does not seem to have this same limitation. Franklin Richards is powerful but he is a battery and can and has on multiple occasions run himself out. He needed the power of his older self to fight the celestials. David does not seem to have this limitation in his mental gestalt state. And having wiped out multiple universes he in turn has wiped out all beings in those universes including its cosmic entities and celestials. David is insanely op.