The idea of ​​paid relays seems like a good idea, but here is a problem: suppose in the future the number of nostr users reaches millions and the number of notes reaches thousands of millions of notes, in such a situation, what is the cost of run and maintaining a relay?  I don't think anyone can offer a free relay in this situation.  As a result, I think that nostr will move towards paid relays in the future, but won't the paid relays lead to the centralization of nostr?  Most users try to choose the best relays and the best relays will be the biggest relays.  As a result, we may have a few large premium relays in the future that act like a bitcoin pool and censor users' notes if they want to.