It makes absolutely no sense to use versioning in regard to the web, because there is no central entity that updates all webpages to a single new version. Moreover there is no clear definition of the meaning of "Web 2.0".
To an extant I agree but there is a marked difference between the web before and after pages were able to communicate with a server without forcing a reload.
These new features (XHR etc.) aren't available in older browsers, it is a marked difference in the capabilities of browsers.
Not all pages make use of those features (thankfully, it often is overdone) but the ability is there, and that's what makes it "Web 2.0" - it is radically different from the days of IE/Netscape 4.0 browsers
I don't like all the changes but it is a radically different user experience now.