I agree with most of what you're saying, but:
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forgejo is working on federation. They've been working on it for a certain amount of time by now, but I do think we can expect some concrete version of what you describe in terms of community to materialize in the next decade as long a people want it and are motivated enough
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when talking about code that is stored in a version control software that supports decentralized state (git, mercurial are the 2 I have working knowledge of) the "easy" fix for low bus factors is to just fork/mirror the software you want to see continue to exist. Source code is not that voluminous, I would be surprised if [the collective we] can't manage to store multiple copies of the sources for software we deem useful. It's a question of changing habits, not finding some miracle tech
Of course, habits aren't necessarily easy to change.