This is generally a massive problem with the fediverse in general — even Mastodon, although you can transfer your identity (i.e. if you link your old account, your ‘followers’ will follow you, and people searching for your old address will find you at your new one …) there’s still no way to transfer your content.
I’ve always got the vibe that the fediverse is heavily skewed towards privacy/temporary/“no permanent presence” folks, so it’s very low on the priority list to ‘fix’ that … but for a hoarder, digital archivist, quantified-self need like myself, it’s horrendous and scary.