Thanks for clarifying. I think the issue with relays blocking spam is you only get the benefits if you limit yourself to those relays. I don't think network islands are good for Nostr in general and for their users in particular.

I think that there are many other ways in which relays can provide value, like being reliable, offering data retention services, not banning you for your speech, etc. 

Ultimately relays are back-end databases and there's only so much coin someone is willing to fork over that. A model where relays are free is feasible, they're very light on resources and there are examples from the past where this has worked, see large IRC networks. IRC servers play no part in filtering channel content either.