I think Linus is at fault here. Having a PR process that requires sending a patch over email, email threads done with super old mailing list software, etc instead of the more modern workflow of using forks, PRs in a web UI, etc is tedious and annoying. Making kernel development and maintaining more accesible to younger people would increase the pool of potential maintainers.
Is there an established open source project out there that originally used a mailing list or some other review software like gerrit, reviewboard, or phabricator, that transitioned to Github or Gitlab and actually increased the number of contributors and maintainers?