IMHO, it's unlikely anyone with an iPhone uses it "purely for SMS usage." That would mean we have an iPhone owner who only receives SMS from services, only sends messages to people with an Android device, has gone through the trouble to deliberately disable iMessage messages or lacks a data plan of any kind.
I agree with you. The problem is that we conflate SMS and iMessage usage in the Messages app. Most people do use Messages for SMS-like messages (meaning not for exclusive iMessage features). E.g. looking at my message list: at least 40% of messages I receive are alerts, payment confirmations, appointment reminders, etc. These are SMS messages in terms of their purpose, even if some have the blue bubble and whatnot. Messages is a popular app in the US, but what we need to look at is how popular it is for specifically iMessage-exclusive features, not as an SMS client.
