It is sad, though, that you have to rebuild again after something like Facebook falls out of favor. Contacts get lost. I guess people will move to newer technologies than Mastodon as well, but so long as the concept is valid and the software maintained, transitioning to another instance is a feature of the system. I hear it has its issues but in general, you publish some redirect on your old profile (there's a button for doing it in your account settings) and then all followers will switch to the new location. You shouldn't have to start over from scratch in a normal case as migration is a foreseen situation