Companies will keep using GH for a long time because they seem to be really tolerant of outages (and have a massive switching cost depending on how much of GitHub they use outside of git).
Smaller teams/solo devs much less so.
Isn’t really anyway to coordinate it ahead of time, it’s more an emergent bottom up thing than a “all devs agree to move to X” ahead of time.
Now there is absolutely a benefit to co-locating issues and PRs with your code in the same git repo. In a way it becomes much simpler to move your entire repository and all activity when everything is in the same .git folder.
With Tangled your code is stored in your code repository and issues and PRs in your data repository. On the other hand the ATProto stuff is what means your project becomes discoverable and that other users in the network automatically can interact with your repository (likes, issues, etc).