Every web page with a menu somewhere hacked out of nav and ul elements and some css would gain by including some application-oriented markup at their top level and restricting the text-oriented markup into the text area.

There's not a clear separation between "text" and "application" on the web. Nearly every site has both.

I disagree. You're right that there is of course no clear separation (the opposite also being true: most complex web applications will contain text content), but the differentiator is when the application side becomes large and complex enough to warrant a change in paradigm. Doing that for a little blog menu is overengineering.

Every page starting at the application paradigm, and explicitly changing when displaying text would fit almost all the cases better than application and text markups that don't talk between themselves.

I don't think that's true at all. It would appear that way if you're only considering the public, "user space" internet. I've done nothing but build web applications in the enterprise sector for the last several years, all of which could not be "just pages".