I copied my personal projects to GitLab today and that was very easy to do but I'm not sure what to do about the GitHub version for the one that had a little popularity.
A fork between services becomes more complicate but not so much: git pull from GitLab and git push to GitHub or whatever. Everybody knows how to do it.
Right, one loses the "fork" link between the two repositories but really, who cares? We're writing software for work or for a cause, we shouldn't be playing a game of collecting stars and forks.
For the stars, they presumably did it to get update notifications. I guess they'll get a notification if I update the README, and they can decide what to do about that. I'm making things just a little bit harder for each of these people though.
Forks are even easier to maintain than you're describing. You just need to do a change the url for your upstream. The issue for me is that I have several forks of other peoples projects and in almost every case it's because that's required to make a pull request. For each of these projects that moves, they're multiplying the small effort by the number of people who need to move their forks and/or learn how to make pull requests on the new service.
The forks thing could be tackled. With a notification, something like I forked you. Or a code search engine which is clever enough to detect and link repos.
The user management gubbins, is the normal irritation of user management and access control.
The stars part is already a bit of a horror across all software - not knowing how to gauge the popularity and quality of a project. Nevermind searching for it.
I'm not the biggest user of these silo repos, but I do appreciate that thin barriers to entry, and improved usability or a good usability wrapper, is charming.