Running a git forge is easy, I find the social coding/network effect the stickiest part of GitHub. I want my code to be discoverable, and I want to discover others' code. I wanted to leave GitHub but I'm still stuck there. What it would take for me to finally leave is some kind of federation of the social features, so I can port my stars over, see what others are working on, etc.
At some point the whole internet switched over from a project-centric to a platform-centric mental model for most people. It used to be "I want to talk to these people. Oh, they're on IRC" and then it was "I'm on IRC. who can I talk to here?" and while that might not be so bad for IRC, it's catastrophic when it's proprietary platforms with lock-in.
The main issue though, is that I won't star a repo on an alternate forge. That is the main reason many projects will only consider Github - if they have no revenue they must have traction, and GitHub stars are a good substitute for meaningful measures of that.