This has been going on for a decade, at the beginning it was projects moving to Gitlab now there's a lot of alternative projects but GitHub is still the only one that counts for discoverability. This is a very small minority of projects that move away from Github and it's way too early to declare GitHub doomed.
Github is fine for discoverability but as a development platform I think it's going to die. Public issues/PRs are a cesspool now and going to get worse, and agentic workflows are going to drive companies to want to hide how the sausage is made. People will gradually migrate to alternatives and mirror to Github while it remains relevant.
No different than everyone talking about the next “iPhone Killer” when someone other than Apple releases a phone. Although, I think that rhetoric has largely died down.