What Apple could have done, for sake of clarity, sanity, and good practice is to handle SMS using one app, and handle iMessage using another, *separate* app.

The problem is not that iMessage exists, it's that it operates in opaque and unpredictable ways, mixing SMS and iMessage (and now RCS) communication in a way where even more tech-savvy users do not understand how it works (first-hand experience - had to explain to someone why their images are super compressed when they send them to me, but OK when they send them to their friend with an iPhone).

And now it's the same with RCS (Android-iOS). I send person A an image, the conversation switches to RCS. They use the "automatic reply" when I call them, conversation switches back to SMS. With person B, the switching between RCS and SMS is even more unpredictable.