The author of this article was also the CEO of Beeper. They did just that and released an iMessage client for Android in December 2023. Apple proceeded to ban users of that client, launched a smear campaign against the company and implemented countermeasures until Beeper gave up on the whole endeavour.

Apple has lots of options at their disposal to frustrate any attempts to reverse engineer their APIs, and have shown they're willing to go above and beyond in defending their walled garden. If all else fails, every Apple device newer than 2018 has a secure enclave and verified boot, so they could just enforce an encrypted channel between the enclave - which will be able to attest that the device is running latest iOS or macOS with all DRM measures enabled - and iMessage servers. The only reason they don't do that already is the number of users on older devices, but that number gets lower and lower each year.

i wasn't aware of the Beeper affiliation that's helpful context. a good college try.