It just needs something to close the asymmetry between the 0-cost nature of sending an event and the expense of relaying and storing it.

Spam is already a problem at the relay level and we are beginning to see some experiments with defense.

IMHO nostr will solve this kind of problem by figuring out how to get sats from end users to relays and other nostr layers in a way that scales proportional to their consumption of resources on other layers. It's a non-trivial problem, which can go unsolved for a period of time, but the solution is the difference between nostr eating the world and not.