I guess running a relay is a bit like running a Lightning node. Most people won't do it, but enough will, that everyone can use theirs and pay them (with love or money 😅 ).

A database is more like transacting on-chain. It's more persistent and secure, and allows for more complex data analysis, but costs more and is a higher hurdle because you're writing to/from the core data storage system.

I expect that there will be more LN nodes than Bitcoin nodes and more relays than Nostr-DBs, but I see viable uses cases for both.