Only one party can keep a secret. And in fact, there is a solution, device specific key (only that device can decrypt the DMs) and same for the sender. If each client on that specific device generates a random priv/pub keys that are shared for the purposes of DM encryption, then leaked nsec (your nostr identity) does not compromise your DMs. Lost device does, but that is a different ballgame. If people are OK to lose their DMs with device loss and only (migrations can be solved), then we will be in a much better position 🐶🐾🫡