Earlier quoted context omitted.
Was that really a concern in the US? WhatsApp and iMessage require data plans, and as far as I can remember these have largely included unlimited SMS as well. It was definitely a factor in the EU, though: SMS still aren't free on many prepaid plans there. WhatsApp was the first popular application supporting unlimited messaging on mobile phones for many.
I'm 39 and SMS not being unlimited was a huge thing. I never thought it would be unlimited, when that happened (Suncom? I forget who started it) it was huge. This was a LONG time ago though, long before WhatsApp. Probably 15-18 for me. I remember a lot of conversations in high school going "My plans maxed out can you text $friend?" IIRC you had to specifically turn off receiving texts or they'd charge you for each on…
But the question here is: Were there any (reasonably popular) plans that provided data for smartphones, but not unlimited texting? If not, that couldn't have been a factor in the adoption of iMessage in the US.