What is your opinion of a hybrid system? Some relays could be free, others advertising sponsored and other paid by users for access to specific info or ad free notes.
Free Relays owned and operated by ad companies and data farms would thrive. It's their business to gather intel so why not host and serve it in order to obtain? That's the point. I think you only have to pay when you get cancelled or censored. Let others get dirty hands. Free relays should be pitching to these people! https://thesocialshepherd.com/blog/top-advertising-agencies-uk
I would expect that people who make money on Nostr will be interested in a good infrastructure. So if McDonalds sells burgers on Nostr they maybe want to make sure that there are some working relays. There could also be some enthusiasts like the people on the fediverse right now. Third option I see are payed premium relays which offer special services.