I think the full solution should not trust relays about how much they are storing, and include a proof of storage/retrievability/data check.

I imagine clients can even keep a Merkel or some hash, and even store it as an event, linked to that relay. And a nonce check or similar can help a relay prove they are least storing those event ids. Maybe a way to expand to covering whole event too, without the client storing all the data.

One edge case is what if someone broadcasts your event. Who pays? And can someone broadcasting events on mass trigger relays to ask for more money from the pubkey, who didn’t broadcast or publish.