There's no incentive for relays to serve ads, because clients will only serve events for people you follow. Unfortunately relays need revenue the most since they are the lifeblood of the network, and are actually consuming resources, while clients are basically free after the code has been written.

The way ads can work is if *clients* inject them into the feed dynamically, and then only clients will benefit from this. However this will disadvantage the client in the battle to be the best Nostr client.

I guess you mean there's some imagined cooperative way we could produce community-driven marketing in a way that makes people feel empowered. I'm cautiously optimistic someone could figure something out.