This strengthens my conviction that federation is a bad architecture for something like Mastodon. A fully distributed system, urbit being the easiest to try right now, can't stick someone with the responsibility to keep a bunch of other people online. It can't stick those people with the responsibility to move off the server. Each user runs a server process, locally or on a remote machine. If any of those goes offline, all the services and data it was providing are gone, but no other user accounts are affected.
Federation works fine for Matrix, although I still think the full peer architecture will dominate long-term. It's less disruptive to something like chat to switch user names because a homeserver shuts down.
Mastodon instances get linked into, and all those links are going to break. Running a redirect for those URLs to the numerous new account homes is impractical given that a lack of time and commitment to server maintenance is the issue.