Depends on who you define as 'you'. If you are 'jbb67@mastodon.social' and mastodon.social goes away or kicks you off their server, there is no more 'you'.
You would lose all subscribers, likes, comments etc.
Then you would have to find a way to prove to 'your' old friends that jbb67@someotherodomain.com is the same person as the person that formerly was 'jbb67@mastodon.social'. And have some of them follow you again etc.
The same process as if you changed from Twitter to Facebook.
you make this sound like a big painful process. i have lots of alts on fediverse on servers running difference AP and OStatus implementations and i've moved my main many times over the years. it really doesn't take that long for people to realize that you moved and what your new main is. hell, i used to update my avatar and display name daily while bouncing between different alts and people could still recognize me.
based on my experience on fedi, i think you are exaggerating the fragility of the connections you make on social media. clicking "follow" isn't a lot of effort and people will do it when they recognize your voice coming from a new source.
What you note is not as painful a thing to most...But I'll grant you that to some it still is a thing. Here's where owning your own domain name helps avoid falling into this type of trap.
Having your own domain name represents YOUR identity/persona on the web - nay, overall internet in fact. The focus here is on the portion AFTER the @ symbol. Yes, unfortunately for now, owning your own domain - for the purposes of interacting with other instances and people via these federation-type of technologies - also involves the overhead of hosting your own mastodon, gnu social, pleroma, ActivityPub server, etc. But in the future, it could be that the underlying mechanisms could be made much easier and cheaper both to use and to switch/jump...and for the most part you only need to hold onto your own domain name...and can switch to different underlying mechanisms/technology/providers/whatever...but the top-level identity is preserved as you cross to other instances, etc. Within the indieweb community - a community that supports efforts like ActivityPub, etc. - owning your own domain is usually the first step...because it represents the formal establishment of YOUR identity. An identity that you control, and where your friends/followers can always know and follow. I imagine in the future if more people adopt the practice of obtaining and sticking to using ONLY their domain name as an identifier, much of this will be easier and some it simply a moot point.
The process if you changed from Twitter to Facebook involves everyone you know also changing from Twitter to Facebook. The process of changing from mastodon.social to social.coop does not.
There is currently limited support for account migration in Mastodon, and ongoing discussions about how to improve it. It's not at the level that it's at in Hubzilla, but it's not a lost cause, either.
