There are quite a few well funded companies now that use forks of VSCode: Google (IDX, now Firebase Studio), Cursor, Windsurf (especially if they are bought by OpenAI soon), GitLab etc.

An alternative marketplace already exists with open-vsx.org, but these companies need to fund the creation of alternatives for the proprietary extensions (C#, C++, Python, maybe others) that can be used by all forks.

A pretty good reason to switch to vscodium because monopolies, even open source ones, are never good in the long run.