Not the best way to look at it. A lot of developers today work primarily on github because it's not possible to interact/work in other projects from their siloed forges. When gitlab implements AP, things are no longer siloed and therefore there will be a percentage of (current) Github users who will free to choose their forge independently of network effects.
It's better to ask "how many projects are stuck to Github exclusively because of collaboration tools, and how many of these would be able to leave Github once more Gitlab/Gitea/Forgejo are able to interoperate? How much of its customer base can Github afford to lose before being forced to give up their monopolistic strategy?"
It's better to ask "how many projects are stuck to Github exclusively because of collaboration tools, and how many of these would be able to leave Github once more Gitlab/Gitea/Forgejo are able to interoperate? How much of its customer base can Github afford to lose before being forced to give up their monopolistic strategy?"