I like the idea of long lasting embedding bots more than the one of specialized relays.

A long living bot attaching embeddings to notes could build a track record of signed transactions and become a trusted npub of the nostr community. This doesn’t have a lot to do with the relay (except maybe retrieval of notes which requires a separate filter implementation).

To make an analogy:
Prefer trusted librarians over a good library building.

This all goes very much in the topic of DVM. The clients should be diverse and heavy. Not the relays. 

And most important:
We should not forget that bot npubs can be long-lasting as well!