You don't need to involve the server in generating the UI at all. You can have a static index.html, that bootstraps a javascript environment from static js files, whose first order of business is to contact a web service to fetch the initial content and configuration as JSON data. This makes all content static except that which is truly dynamic, which offers amazing CDN and caching possibilities. The JSON data can be cached in local storage, for easy offline support without constantly revving manifest files.
Once you get at that point, the dev experience starts to look a lot like traditional desktop client-server app development. Except desktop apps are built using components instead of templates, components that combine rendering and behavior into one encapsulated entity that you can treat like a black box. There's nothing preventing a web app from using that same architecture. In fact, ExtJS does just this.
But I don't think it stops there. I think we're going to get meta-configuration languages like mxml to send the layout of these components to the javascript environment, which will, essentially, mean that we're building another browser inside the browser. And so all software evolves until it can render a web page, including web apps.