...I don't see how a lot of them will continue existing past the first year or two. Thankfully Mastodon allows you to move between instances and also take your followers with you <snip> which can help if your instance develops the wobblies.

One of my friends had to migrate as the Mastodon server he'd joined is shutting down.  As you say, you can migrate your followers/followees with you, however you can't migrate your posts.

Gavin