When I hear someone say "I wish I had an iPhone right now", it's usually because they're either (i) lost, or looking for something, and want access to Google Maps, or (ii) in the throes of gadget envy. The App Store addresses (ii), but not with any single app -- it's just the idea that there are thousands of fun toys that only iPhone users get to play with.
So there are a couple of killer features, but nothing that can't be replicated on other phones. Not necessarily as well, but that's not the point: without exclusivity, the iPhone will not get the same kind of dominance Windows has.
If you polled a random user of, what are the chances you would find an app that they will get very pissed off at it's disappearance?
What are the chances they will actually skip platforms?
On Windows or OSX, I think the answer is high. On iphone, maybe not.
It is still a luxury for most people - "oh cool look at this app," but it still comes down to those that can afford an iPhone. I can imagine there are many family squabbles involving parents that bought their kid an iPhone that racked up an App Store bill the parents didn't originally intend.
iPhone is not a business phone overall - and therefore doesn't have "critical apps".
I really doubt this is true given the lack of any killer mobile apps. I don't personally consider any of desktop apps in the list I gave 'killer', but fairly sure they've sold lots of computers.