That suggestion that people buy iPhones merely because of iMessage is outrageous to a non-American. While I might certainly be one of the reasons, I doubt it’s the reason, unless the user is 12.
Not being able to communicate in large group chats with all your friends is social suicide for many people. You won't get 10-15 people that have used IMessage for years to switch, so yes imessage is a primary reason to use iphone.