You said: People don't go "I really want a walled garden".

So you admit after all this my exact point in the first place. Almost nobody is deciding based on this because there are thousands of other things that are either more important, easier to understand, more talked about in marketing and reviews, or simply because they've never even heard that some are walled gardens and some aren't. Clearly someone who has never had a smartphone before wouldn't choose based on specifics of the App Store when they're probably trying to figure out how to make a phone call.