Sachin, isn't that true about Posterous too?
While I agree with the points you make about the weakness of the web, you are missing one key point.
The web makes it possible for the end user to stop thinking about the OS and installing stuff. If we had to move to the model you are suggesting, people would have to start choosing browsers. It would be sad if I couldn't use Posterous because it doesn't work on the Ubuntu version of Firefox.
Besides, considering most people don't even know what a browser is, why would you want to make them install 5 different ones? It will only bring back the IT department.