@Jonathanglick a standard migration process already exists for moving your follows/followers to your new account, it's just that pretty much everything besides Mastodon and Pleroma (IIRC) don't yet support it.

Moving posts too would be nice, but significantly more complicated. I'm not a dev so I couldn't really speculate how much more difficult that would be, but IMHO for most people the current profile redirection and leaving posts on the old server is probably fine. It'd be nice to be able to publish old posts somehow in case that old server goes down entirely, but I think that's a bit more of an edge case that most people won't realistically be too concerned about. Moving your following you've amassed maybe over the span of years is far more critical though, and it annoys me that it is so uncommon outside of Mastodon specifically.