To me the biggest not-easily-replaceable value is the "Hub" part of GitHub. It's easy enough to stand up your own Gitea or Forgejo instance for mirroring personal Git repos, set up your own GitLab instance for more sophisticated collaboration and CI workflows, etc...but the discoverability GitHub provides in its current form, since the vast majority of repos are hosted there (or at least have some kind of mirror there) is unparalleled.