I have never received a single spam message through iMessage, but regardless. If they have to keep buying new phones all the time as they get blacklisted, in order to keep spamming, the economics of spamming changes.
That does raise the bar a little bit but it's probably mitigated by the combination of targeting higher spending users and the additional trust iMessage provides.
The difference between being able to send unlimited messages for free, and having to get a new phone every hundred or so texts is pretty large. Free vs 1 dollar per spam text.
Nothing, that's why iMessage is full of spam already.
What scammers do is buying boxes of old half broken iphones and they turn them into relays exactly as you said.