Afaik relays already have an option to include ads. Clients however are not required to display them.

The only way a forced ad model works on Nostr is if you control both the client and the relay.

Paying for content and publishing however is slowly becoming more and more normal. Think Twitter Blue (or whatever it is now), Youtube Premium, Patreon and similar services, and even paywall newspapers like New York Post and so on.

The crucial question is whether we can provide a better service for a given price than competition.