One big advantage GitHub had was that it was an easy place to find contributors. But now you don't really want a lot of driveby contributors. This makes GitHub alternatives a lot more viable.
The network effect is dying.
Vibecoding idea I don't have time for: Take SQLite's version control system Fossil (where all the code history and issue tracking and a bunch of other stuff live in one sqlite file) and figure out how to make it work really well with agents.
Maybe it does already - it's old and there is probably a good amount of training data on it, and I'm sure it's amazing if the SQLite guys built.