Several business models for relays exist, such as subscriptions. nostr.wine is arguably our most successful paid relay with several relay types in their service catalog. Another would be services rendered for relay access as a packed service offering, such as nostrplebs.com and others.

Additionally, you don't have to run a large scale relay. Small community relays are the future thanks to NIP-65 outbix model. That's the future and many applications support it. Damus and Primal are the last major holdouts. Once they move over to this model, which they've previously mentioned doing so, the smaller community relay models will make more sense for everyone and decentralization goes up as a nice bonus.