It's definitely not a no-brainer to replicate every post/comment in the fediverse to a single-user server where the user is only interested in reading 3 of the communities. That would induce a massive federation/replication overhead both to send and receive posts and comments that no one cares about. Lazy replication that only kicks in when someone asks does admittedly feel weird on a brand new server, where everything is new/empty... but it makes a fair amount of sense if you think about what's going on behind the scenes. I'm sure a person COULD write a script to subscribe the admin or a dummy account to a seed-list of popular communities to get things started, take a crack at it if you want. It's not clear to me that it should be the default though, as this doesn't make sense for single-user or very small instances, they should start empty and decide what they want to subscribe to.

I do think that a useful middle-ground would be to populate the LIST of communities. The all-posts/all-comments feeds really do need to be constrained by what communities people are actually subscribing to. But the number of communities is much smaller, and their rate of creation/change much lower. I think it could be reasonable on federation init to ship the full community list across the wire even on a big server that hosts thousands or tens of thousands of communities, and then eagerly ship community creation/deletion events across the whole fediverse. That seems like it would be a manageable volume of information, it would enable every instance to have a useful /communities/ page from the moment they federate, and it would make subscribing to remote communities much simpler. Right this minute the devs are trying to fast-track performance improvements to keep lemmy.ml from falling over due to the new influx of users though, and that's good/important work to prioritize now.