I will add that I would expect to see embedding-only relays for search purposes.  The relays would be write-restricted so embeddings can't be forged.

To make those workable, they would probably be a paid service, but they'd be much less vulnerable to forgery.  Such services would likely need to employ bots to generate embeddings on new content when it is published, but as we saw with ReplyGuy, it's not hard to make bots that can interact with all new content.