Relays can't filter by content what is encrypted. So if you remove all the meta data including the sender, you won't know who could even detect spam in the message. Denial of service spam could post to channels that don't exist so to say and relays would store the garbage?

This content would be bound to relays that get paid for its storage probably by the content's author, which is not automatically wrong but still a few steps away from where we are so far.