If you look at the early days of nostr, paid relays were a potential sustainabilty source. Its one reason zaps are integrated. The Ad model has bad and anti-social incentives that we all could have avoided by paying for services. Imagine the lack of spam we could have had if each email cost a cent or something. Nostr's biggest threat is the same threat every other open protocol faces, capture.

Xmpp was embraced and capture by google.

All this could have been avoided if its users had paid their ideologically aligned core devs to make and focus on the protocol, instead of ad backed whales coming in. This is now super easy with zaps, the way nostr:npub10qk5zpmhv7rspp87shajf7d24yrf4lyr7w0m25wv9w78grs4k0sq0gq8pcand others are supported with Damus.

People like nostr:npub1sg6plzptd64u62a878hep2kev88swjh3tw00gjsfl8f237lmu63q0uf63m and the way he supports these projects is also good. He empowers devs and open networks in a hands off way, for the sake of society.

Tldr; we don't need ads to remain sustainable, but you are always free to start up an ad or subscription funded relay that curates the feed.