The nice thing is that nostr is truly decentralized, so it doesn't really need to "scale". You can self-host or uncle jim, in which case volunteer nodes are fine. The ones that most need a business model are indexers like purplepag.es, or hubs that new/non technical users can use to easily get started like relay.damus.io, and those can likely be subsidized by businesses built on nostr. Then there are services like nostr.wine, which charge subscriptions. There's also been some experimentation in usage based pricing facilitated by ecash. So I think the problem isn't currently solved, but there are lots of leads to pull when it becomes more necessary. And it will, as spam of various kinds becomes more of a problem