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.
I always stayed with GitHub because it just worked the best. GitLab was slow and janky. gitea and its various forks lacked features and felt a step backwards. Sourcehut workflow is far too opinionated for my liking. Don't even get me started on GNU Savannah.
Some parts of the Free Software/Open Source crowd has always bemoaned the rise of GitHub, "because obviously you should use Free Software, its your ethical duty!" Most people just use what works best, including many Free Software devs. There is a loud minority (even louder in bubbles like HN) but for most people it's just one factor out of many, at best.
The reason GitHub became dominant is fairly simply: it just worked the best. Doesn't mean it was perfect (remember how long it took for line numbers to not be copied from code examples?) but the alternatives were even worse.
It's interesting to see how badly they're messing it up. You'd think that making a new react-based frontend for a fairly uncomplicated issue tracker wouldn't be too hard, but seems like it is. Some initial bugs after a rewrite are normal, but ... it's been a year? I still regularly just see closed issues in my issue overview. The back button is basically just broken. These are not obscure heisenbugs: these are bugs you find after using it for five minutes. The entire experience is just so janky.
I don't think Github is dying at this moment. I do think that the regression of UX quality is a necessary pre-condition for its death. Like many things its death will happen "very gradually, and then suddenly all at once". Sourceforge once seemed omnipresent and that changed very quickly.[1] But who knows where things will end up in five or ten years?
[1]: I'd like to pre-empt the inevitable "that's because of the adware" comment that someone always seems to post: that's a false history. The adware happened well after it already lost its position and was the desperate attempt of a declining struggling platform for income.
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.