To support decentralization and spread, should lemmy.world close registration at some point to prevent a performance overload due to too many users? Of course, if registration is disabled, there could be a hint placed somewhere near that from other instances you can interact with content on lemmy.world just like you had registered on it. There could be a link to join-lemmys instance overview.
The danger in randomizing servers is that some smaller servers not only have less than 99% uptime but are also just run by random regular people who couldn’t handle the increased load and/or have no desire or ability to keep the servers running long term. It could maybe work if the randomization occurs from within a vetted list.
Account migration is a feature that has been noted for the future and would indeed be very important since it would essentially make the entire network bulletproof. Being able to move instances and/or link accounts across multiple instances would create the necessary redundancy and reduce fears of choosing a smaller instance as home.