> If the instance you’re on is compromised you will lose your identity because private keys are stored as part of the instance. No private keys, no identity, no portability.
I don't think it's quite so bad, although of course a server vulnerability is bad: you can have an email account as a backup. That's the way many services have kept secure: you can invalidate the old password after a breach and email users a new one. In the worst case, you could try to prove to moderators you're really the account owner. However, the most important point is that it's just an account: you can just create a new one, and be mostly fine (after letting people know you've moved). In the case of the breach mentioned before, a notice could be left on inactive post-breach accounts.
There's not really too much data you'd care about that isn't public already, and you can do regular backups in case there is (private messages, I guess?). It might be nice to add some auto backup functionality to some cloud service I guess.