I would expect both things to occur. Web of Trust among relays and inter-relay attacks.

Then miners can spam relays for almost free? 🤷 Miners could sell that capability ... Anyway, I'm sure you can do better. Use Cashu to reward the first m of n relay operators in a transparent way. I assume, mints can execute arbitrarily complex logic so it should be no problem to have a way to reward the first 5 of enumerated 15 relays for picking up a note. People would have to track if the relays take the money but delete the reward or if they retain the event for a reasonable amount of time but this way at least the relays would get directly rewarded for building the infrastructure. To avoid centralization, we would need ways to spread the load, favoring less central relays but with the outbox model, people already can use their own relays and have little benefit of using centralized ones. @calle

Nice try. Now every nostr user is obliged to have bitcoin and subsidise the miners while the relays get? Decentralised personal and group relays are probably the best solution, you can't spam my relay, and I only hook it up with others. The word "relay" is another poor nomenclature choice. It doesn't much relay as store and forward, and it doesn't do that to the extent that say SMTP MTAs did. The motive for this, enriching miners while they do nothing and others do the actual work looks like an indirect subsidy. Fortunately it's not mandatory and if you choose it, all strength to your hand. Best outcome is this isn't yet-another-login-protocol kind of problem that complicates and reduces utility. Coming from the home of Electrum, that's no surprise.