Without bindings to hosted mobile functions, like sensor access, etc. you are just making a website. So that's an argument for why mobile apps won't go away.
90% of the apps I have seen people proudly swag around with on their iPhones and Androids could simple be rewritten using html5+css3, very very few of them made use of any mobile-device specific sensors or drivers.
You're 100% right from a technical standpoint, but it's a bit hairier on the UI design side.
A huge strength of iOS is that most iPhone apps look and behave like most other iPhone apps, and that's because of the coherency provided by the UI framework and HIG. Asking developers to write their own CSS styling for widgets would result in a much more fragmented experience from app to app, which would detract from the platform as a whole (on top of a whole other slew of tiny UI touches added to UIKit, like the surprising complexity of the view-to-view transitions in CoreAnimation, that would be lost on 99% of app developers if asked to develop their own animations).
One of the most interesting things Apple is working on that everybody seems to ignore is iAd Producer. They've basically created a full-fledged HTML5 app-building tool that recreates a large part of UIKit in JavaScript, but it's currently only good for iAd development. I'd love to see what would happen if they would let developers release apps built with the tool.
That's true but I'm also seeing a trend away from using Apple's native widgets in a lot of apps. I think users have seen them enough now that they see standard UI elements as generic and indicative of a boring or low-quality app. The leaders mostly seem to have rolled a lot of their own look & feel, even if it's just image-backed UIButtons.
You're already starting to see this. For example, this web app: http://html5photos.appspot.com/ when viewed on recent generation android phones utilizes hints in the markup to trigger the phone camera for uploading. I think we will see this sort of integration continue. Will it be a pain dealing with proprietary markup across browsers? Sure, but it will still be faster than writing multiple native applications.
It's been quite a while. Apple started down that direction, but then the app storm hit and that has been the end of that. And HTML revs much much slower than any phone API, so the native experience will always be superior.
This is a key advantage of native code, but it is being eroded by the rapid development of WebKit and Gecko. Browser makers are driving the environment faster than we initially expected, with the tradeoff of potential fragmentation.
