Obviously I don't have exact numbers, but I am absolutely sure the huge majority of people buy iPhone because it is iPhone, and they have zero idea about this whole openness vs security debate. Apple won the market with marketing, not with proving that openness doesn't matter
For a business that makes it's money from consumer spending habits, I think this is a perfectly valid argument. Apple isn't a utility company or something the users are locked in to. If they decided openness was more important, why are they still buying so many iPhones?