You can easily migrate your repo to a different service, so having lots of developers rely on GitHub isn't really a big deal. Many large projects have GitHub mirrors.
I'd say the biggest issue is that Git doesn't include better built-in support for issues, wikis, PRs / code reviews, and releases. Compare that to Fossil [0], which lets you bundle up everything into a single file. If there was better built-in support you could migrate everything more easily to self-hosted alternatives like Gitea [1]. Regardless, it's possible to migrate manually, even if it's a bit more work.
I'd say the biggest issue is that Git doesn't include better built-in support for issues, wikis, PRs / code reviews, and releases. Compare that to Fossil [0], which lets you bundle up everything into a single file. If there was better built-in support you could migrate everything more easily to self-hosted alternatives like Gitea [1]. Regardless, it's possible to migrate manually, even if it's a bit more work.
[0] https://www.fossil-scm.org/index.html/doc/trunk/www/index.wi...
[1] https://gitea.io/en-US/