I wouldn’t be surprised if the response was a Chrome-esque “drown the competition in feature velocity” approach. iMessage could add new features (with totally necessary payload shape updates) fast enough to prevent any integrations from working well and competing effectively. Throw in some EME-style approval-requiring binary blobs to enable decryption and a dash of CSAM-style regulation compliance and open vs closed won’t really matter (like we’ve seen on the web so far).