Well, yes, and well no. Browsers are, no doubt, getting more and more powerful and obtaining more power over the host system, but did you know that with all these new powers there are more security features built into browsers than ever?
What if web apps were permitted to access a sandboxed part of the file system, somewhere they can save a file for instance so that the next time the user visits the app or website they could load the file. For instance, with my ClickIt project this would be very useful. But how useful would this actually be for an everyday website? That I guess depends on the focus of the website.