You bring up some good points. I agree on the risk, even though I'm a fan I find federated tools harder to get started with.
I agree git is decentralized, but services like GitHub are not. They're more than just hosting code. They're issues, wiki's, CI/CD, peer reviews, etc.
how do you control who can and cannot make changes to your codebase?
I'd image it's the same as now. Except now you could say @everyone@that-server is cool and can contribute, or @those-guys@over-there shouldn't even be allowed to see this code.
How do you ensure you maintain access if a server goes down?
How do you do this on GitHub?
what value does that provide over the status quo?
I feel like this is the root of fediverse problems. It's easy to send your first tweet, but that first toot takes some effort (I just learned they're called toots).
I thought this was the dumbest thing until I had one. Now I have one in every toilet.
https://www.amazon.com/16-Color-Activated-Detection-Birthday-Gadgets/dp/B07L2Y84K3