Most businesses, consumers, and developers universally continue to ignore the primary reason that iMessage is a closed platform, rather than an app on every platform as iTunes is:
Apple is using device serial numbers for anti-spam, supported by a fully-authenticated hardware and software stack that does not allow user modification. This permits Apple to simply “console ban” any Apple device that spams on iMessage. This makes it prohibitively expensive to send spam over iMessage. They have been doing so since iMessage was launched.
Android offers no such attestation that I’m aware of. Windows, on Pluton, could offer this attestation securely — and that is a key deliverable of Pluton.
It’s easy, then, to predict what Apple’s first non-Apple platform will be: Microsoft Windows 12, only if secure-booted, with Pluton-signed attestation that the kernel is unmodified. And it’s easy to predict how Apple will implement anti-spam: by applying “console” bans to specific Pluton chips by their serial number.
If Android wants to join the party, then Android phone builders need to implement secure boot with hardware-signed attestation of non-rooted-ness, in the style of Apple T2 + macOS or Microsoft Pluton + Secure Boot. Until then, Apple iMessage will remain single platform.
(I recognize that this is extremely unpalatable to device hackers, but the same freedom to modify an OS kernel that hackers desire is also the freedom to spam all users, as we have seen repeatedly with all messaging software platforms operated without hardware-backed attestation for the past thirty years — including email, Jabber, and HN itself.)
(No, I do not work at Apple.)