That's a really interesting idea. Let's ignore the five or ten years it'll take to upgrade the infrastructure for the moment. Let's focus instead on what it would be like to program for the web in this scenario. What kinds of interesting apps could we deliver? And how do we get there from here? (And will it actually take five to ten years?)
Here's another idea to ponder: what does the path to Web 3.0 look like? I'm not sure, but I think iTunes is on that path -- one app that's got the web wired in (CDDB, iTunes Music Store, single sign-on (with AppleIDs), and commerce). Maybe to get to Web 3.0, we need to take half a step back, to Web 2.5 -- the custom client that can deliver an entire rich app in a single framework with (theoretically) a single programming language for both the front end and back end.