The ability for different groups to experiment with different strategies to address the very real problem you bring up is one of the strengths of nostr. I sometimes imagine that nostr is like IPFS except minus 90% of the cool but unnecessary complicated stuff, including the fancy schmancy kademlia network that connects all the relays I mean all the IPFS nodes together.
Discussion
My point being that I think IPFS suffers from the fact that users are locked-in to things like kademlia that are cool, but suffocating. Nostr has done a good job avoiding that sort of trap. Just something to keep in mind when contemplating what to do about relays.