For the largest instances, as long as people are using lemmy in 10 years then they have incentive to exist... People will donate as needed; it isn't terrible complicated or expensive to run an instance.

IME the reason they don't have a "join random" button is that would be bad for smaller instances and a "join largest and most popular instance" would be criticized for showing favoritism and discourage adopting the entire point of decentralized technology. Matrix gets this criticism all the time because they push a default sever in their defacto standard app Element.

As a casually interested user? Just Google what to do because more users will trust the latest tweet or reddit thread more than a FAQ page. You most likely created an email account without going to "email.acme.org" and asking what provider to pick and probably have gone though a few providers without much loss.