I think this is really why it never caught on here (having fallback to SMS by default). I have an unlimited voice and data plan here in Spain, and don't get any SMS included at all. There's not even an option with my provider to have any included. 0.1 euros per SMS, it's kinda crazy. I maybe send 5 or 10 per year, usually some kind of response to an automated 2fa system or etc. If iMessage ever took off I'd always have to be aware of the green/blue bubble thing and make sure to never send a message using SMS. WhatsApp/Telegram/Signal/etc is just easier since you don't have to think about it.
With iMessage fallback is on SMS, so people would be paying for each message sent. And that's the reason why it never catch on.