Github is an ecosystem and its biggest moat is the number of people that use it.

To make a replacement, not only do you have to improve on support for every major use case at a technical level (no easy task, to put it mildly), you also have to make it so compelling to use that Github users will abandon Github en masse.

Someone with an LLM assisted IDE has the theoretical potential to improve on all major Github features. But to make their replacement compelling enough to get folks to leave Github? Not a chance.