Outside of npub portability, again, it seems like you're just describing Mastodon. Federation means different instances can indeed share information, and I guess there is technically a means of user migration between instances, even if it isn't frequently used... But in what fiatjaf is describing, you wouldn't be moving your npub at all anyway!
"Just connect to these three or four servers" just seems to defeat the entire purpose of a decentralized social network. And besides that, the issue is that it vastly increased the legal liability on relay operators, who now 100% have to operate as content moderators.
