Swiss datapoint: my whole extended family/friends circle has iPhones and they only use WhatsApp (and FaceTime for video). I'm not even sure I ever heard the name "iMessage" before.
Nobody I know colloquially calls it that in the US either. The app is called "messages", and people just call it "texting"... "on an iPhone". I don't think most people even realize it is not SMS/MMS.
Nobody knows what SMS or MMS are either.