I received my first piece of spam on iMessage last week (used it since 2011) and it was deleted instantly by Apple. I presume because they identified and removed the account's iMessage privileges.

Any messaging app that permits spam is user hostile. Spam takes up more of my time than I would ever want to give on Whatsapp/Messenger/Text. iMessage has prevented me from contacting precisely zero people. If I were to walk away from my iPhone I would lose nothing after exporting some messages.

The barrier to entry to other potential users of $100s is well worth it for me.