Apple could easily charge $1/mo or $10/year for iMessage on secured devices, with automatic refund and prorated cancellation if no secured device is signed in within a given billing period; and then discount $1/mo if one or more Apple devices are signed in and active during a given billing period. They'd make a billion dollars a month off of secured Android users, without exposing themselves to any new spam whatsoever, and showing Android users that Apple users have a better experience. Win-win for platform marketing and cloud services revenue.

iMessage spam isn't non-existent because sometimes someone tries to spam, gets a few messages out, and then their device gets console-banned. The iMessage "unsend" feature doesn't yet exist in any released iOS or macOS, so it can't be used to hide the spam after the fact.