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.
Well Ruud who runs .world also runs Mastodon.world which is a fairly large mastodon instance, so he is somewhat of a known quantity and has experience running large Fediverse servers. His mastodon server has handled a large population and donations happen through Open Collective for transparency as well. He also runs Calckey.world though that is much smaller.