Rails is just an API
I think this makes sense for web sites, but not web applications. An approach to complex web applications like this, persisting state in the DOM and relying on the server to update the UI only exacerbates the initial spaghetti problem. Aside from that, your interface is only as fast as your network connection, and frankly speed matters.
The caveat in moving state to the client is that it's a huge perceptual shift for developers, with a steep learning curve. But there are some thorough resources out there, and the potential upsides are huge, bringing incredible, desktop-like experiences to the web, without the 'click and wait' interaction that's haunted the request/response model.
So where does this picture leave Rails?