What would you want a VSCode fork to do that can’t be easily done with extensions? (which Codium can run)

It’s more about not reinventing the wheel. A fork needs to have a reason to exist, because it takes significant effort to maintain and develop, and there is significant opportunity cost when that level of development activity is committed to that purpose. If there’s no reason to have a fork, then it’s more efficient to keep all the development energy and momentum focused in one place. And for Codium, that place is the extension repository.

If Microsoft starts actively making the core software worse, restricting or stopping updates to the open source code, tying telemetry into features in ways difficult to remove, or otherwise sabotaging the functionality or features of the non-Microsoft parts of the code, there may eventually be a need for more, harder forks taking things in potentially different directions to get around Microsoft’s interference. But since that hasn’t happened, the non-Microsoft build process remains quite trivial and VSCode remains a perfectly cromulent editor when building it without the Microsoft crap, there’s really no need for any other forks. Codium does everything it would be reasonably expected to do.