> 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.

That statement is false. Primary reason is, that Apple wanted to lock customers in their ecosystem. You can read about it here: https://www.theverge.com/2021/4/27/22406303/imessage-android...

Also attestation doesn't stop spam (see other responses in this thread) and is harmful mechanism that will lead to more walled gardens.