they can be
Discussion
But they don't have to be. And there's the point that the big ones make good hubs for a hub-and-spoke network, but there's no need to read/write directly to them.
Both of these models, in other words, don't work very well, but relays allow us to have a combination of the two. Like a logistics network, but with data.
or you can remove the indirection and directly read/write to hubs and lose absolutely nothing
You lose censorship resistance, decentralization, independence from large relay operators, and on-server bespoke administration.
You also lose a lot of redundancy, but the redundance is all behind a limited set of access points. Walled cities, rather than individual homesteads.
I prefer to have a homestead and visit the cities.