NIP-46 is nice and necessary, but it seems to solve a different class of problems, with a different paradigm. It requires a dynamic "signer" who must be online when the event is signed; since a not custodial signer is 99.99% a smartphone application, this means that events cannot be signed if the user's device is offline.

In the example case of sms2nostr, the user would have to configure a VPS with a signer, and obviously this is not feasible for consumer applications.