There's a number of them

1. Private relays are incentivized to provide good service and spam protection/moderation to their users since users pay them.

2. In other areas where we have many people/companies/markets depending on a piece of software, that software development tends to be decently funded. Bitcoin development is well funded. Apache development is well funded, etc. There will be many many apps built on and relying on nostr.

3. Many apps will or do suggest or require that a portion of zaps which you make through the apps goes to the app developers. This is very reasonable and can make a decent income for devs pretty quickly. I'm happy to contribute to development in this way, most users are too. If this is the standard model, devs have incentive to make the best nostr apps which can retain and attract large user bases.

4. Apps can include ads if they want. I'd rather zap than see ads, but I don't have a problem with ads. Premium features, like you say, is another option.

Keep in mind that at least for note storage, you can fit millions of notes on an android phone, hosting notes and relaying them is not that resource intensive. For other storage, there are many good existing P2P solutions we've had for decades that just need to be integrated to nostr. Every nostr client should route traffic to help the network in a true decentralized, censorship-proof, privacy preserving, P2P fashion.