Not that this isn't nice, but won't it inevitably be the case for 99.999% of users (if they actually get to be large enough) that they reply on some other servi

Not that this isn't nice, but won't it inevitably be the case for 99.999% of users (if they actually get to be large enough) that they reply on some other service (or BlueSky itself) to provide their @domain? I can't imagine that the full solution for account portability requires every single user to own their own domain in perpetuity.