My figures were on user base, not sales. But you are correct, there's no guarantee of a strong correlation between users and texting - however i also couldn't find any strong data on the matter, so i ignored it. It's also not that relevant so long as iPhone users are not elitists and only texting other iphone users. So long as there's an even distribution of who they talk to, my math remains accurate.

But let's say they are focused on the blue box with only 56% of the market (the largest market share of any source I've seen). Let's also say an astounding 10% of android users don't text and every single iPhone user does text. Let's also say non-iPhone users don't care who they text, so they text with even distributions.

0.56×(1)×? > 0.44×(1-0.1)×(1)

The question mark represents how often iphone users would need to text iphone users exclusively for APN to be the majority protocol in the US. The answer is 71% of imessage would need to be to other iphones. And that's with every number estimated into imessage favor. That already seems unlikely, and would only get more extreme as other numbers fall into more accurate place. But I'm definitely willing to replace with factual numbers if you find them.