The 'account portability' piece is bullshit! The way 'account portability' works is by having two separate keys, one for signing and one as a 'recovery' key. You're supposed to be able to use the 'recovery' key to rewrite history if your account gets hacked or some shit.
WE HAVE THE ABILITY TO DO THAT AS SERVER ADMINS!!! MASTODON HAS THIS ALREADY!!
Additionally, if a Bluesky server goes down, their way of keeping access to your data is by STORING ALL OF IT ON YOUR DEVICE!!!!