Why would you want to do that?

You'd break 100% of existing podcast apps; and you'd need people to understand which relays to subscribe to in order to get their favourite shows (there's no "Nostr", just a random list of relays).

RSS is extendable to add new features - and right now is the biggest use of Lightning payments. Nostr is tiny in comparison. And you want to kill that for... why, exactly?