You can run a public, private, paid, community, etc. relay. You choose the model. If you want it to be paid, you'll need to offer services to stand out. Maybe one of those is long term storage. Data can disappear at any time. I run my own personal relay to backup my own notes for as long as I see fit.

Damus purple subscribers pay for all the storage

A lot of people run BTC nodes without any economic incentive. And running a Nostr relay is theoretically is a lot less resource intensive -- at least in terms of HD space....

When you’re a reporter that gets deplatformed and lose years of work that you uploaded to X, that’s the incentive to run your own relay. Notes are stored forever unless your relay supports delete requests. They are stored on the hard drive of your relay. You pay for the hard drive, and you might charge for the service of using your relay.