iPhone will have a market as long as people are tied into "Doing it the Apple way!". That market may disappear IF/WHEN someone comes up with a viable alternative to iTunes that allows the majority of users to access all their data on any device by simple plug and play methodology!
As to the argument about what is better or what will beat what.....Well, think about it like this. IF you start a child on a v-tech laptop it will quite quickly develop the necessary skillset and want to move onto a more technologically advanced system.
The dumbed down tech approach that Apple have utilised to score a march could ultimately be their undoing!
It amuses me when people use logic like this. You presume that everyone who has an Apple device has one because they are mindlessly buying into the Apple hype and want to do things the Apple way. You also argue that people will naturally progress to wanting something that is more technologically advanced.
Neither of these is true. I have an iTouch that I recently sold to a friend of mine. I sold it because I want just one device to carry around instead of carrying around an MP3 player and a phone. I enjoyed the iTouch. To me it was a great media only device that also ran productivity apps. Why does my friend want? Because he wants to do things the Apple way right? No. He does not want/need a smartphone because he doesn't want the extra cost every month. He does however want a mobile gaming platform that is primarily an MP3 player. For him, an Android phone (or any other smartphone for that matter) is a bad choice while the iTouch is perfect.
I have a DInc. It's unrooted and running all the stock apps and Sense UI. I could root it, run a custom ROM on it, upgrade it to Froyo and do all kinds of more advanced stuff on it. I have no desire to do so. I don't want to run custom ROMS, I don't want root. I'm not enough of a tech head to want any of that stuff. The fact that my current phone does more than my old phone does not make me want to do more with my current phone.
It bugs me when people have some idea that everyone who uses an Apple device is somehow a mindless sheep and is not using the device because it works for them, but they are using it because someone told them it was cool. Their needs could be satisfied by something way, way better, but they stick with the inferior device just because.
RIM and WinMo are widely used in enterprise environments. Android and iOS are not. Why? Because RIM and WinMo excel at corporate email and it's only recently that Android and iOS have done so. Even then, both iOS and Android even more so are somewhat unproven in this department. RIM and WinMo are not so popular in the consumer environment. Why is that? Because the needs/wants of the consumer are different than enterprise.
It all comes down to what you need in a device. You should've began your post by saying "iPhone will have a market as long as they make a solid device that appeals to a reasonable section of the market."