I agree, this also doesn’t take in to account that relay use cases can grow. If I subscribe to Medium, then I can access Medium articles. But say I were to subscribe to a premium @YakiHonne relay that content locks certain articles: this will also be able to accept all forms of nostr comms.
If @semisol is correct then costs scale with user numbers, not usage or content size. Therefore paid relays can have a base case and peripheral cases that support other parts of the nostr protocol without impacting subscription profits.
At least that’s how I understand it