What does it mean to "take off"? It is kind of silly to suggest that people do not use web apps. A boatload of people use Gmail and DropBox. I guess if we are asking why aren't they even more popular and why don't native apps start to die off, at least part of the answer lies in the fact that browsers are only starting to support a lot of the standards in HTML5 that will enable true "web apps". Once we are able to get most users off IE 8 and older browsers I think there will be a surge in the creation and use of web apps.