In the Messages app on iOS, how the message was sent is clearly marked. At the the top of the conversation window, above the chat bubbles, it will say “Text Message” or “iMessage”. This line will also appear between chat bubbles whenever the current thread changes how messages are being sent.

But I think I see the miscommunication. I’m assuming that iMessage gains interoperability and thus there are more people who could potentially send spam. But I think you are arguing that SMS would be replaced with the more advanced, interoperable protocol. In that case, I understand your arguments why there would be the same amount or less spam.