Agreed that this would be a sensible improvement to current browsers. But on its own it doesn't really improve the user-experience much. You'll still have to sit there twiddling your thumbs while the page downloads in the background. We need XMLHttpRequest (or something like it).
(Again, recall the disclaimer in my original post: "no, I'm not against all that is AJAX. I'm really just posting about the quoted-and-discussed issue.")
My original post was regarding the concept that AJAX techniques provide a solution for the annoyance of needing to fully redraw a page just to see relatively minor content changes. The AJAX solution to this problem is a hack that prevents the browser from throwing away the current page and starting from scratch. If the browser itself were smart enough to do this when it detected substantively similar blocks of HTML, this hack would not be necessary.
However, this UI benefit comes at a cost -- back buttons don't always work quite as expected, development / maintenance time is typically longer, compatibility is often sacrificed, etc.
If someone were to spend the time to optimize a browser to deal well with loading pages that are vastly similar to their predecessors, all HTML-based websites would immediately benefit (when accessed with that browser). As I had attempted to make clear in my original post, there is clearly still a place for AJAX techniques, as the HTML primitives are, well, primitive, and do not really allow for much fancy stuff (except for everyone's favorite, of course, the venerable BLINK tag).