if there were an onion service keypair derived from the nsec, so that it was reproducible, and launched locally automatically by a client for the purpose of providing a Nostr relay for both the user’s own posts and DMs, it would make nip17 competitive with other messengers on the basis of privacy.

There’s messengers that already do this. Briar, Ricochet Refresh. Of course, it requires you to wait until your client is on at the same time as your conversation partner’s for message delivery if you aren’t using external relays for gossip.

Short of that being implemented, it is unlikely that a fraction of a percent of nip17 users will ever operate a private inbox relay.