I think tiling windows managers are indeed a response to the fact that screen sizes are big enough now that you can comfortably fit 4-8 xterms on a screen and still be able to use them.
Fortunately things are not either/or. I have my xterms set to tile, because working with several machines at the same time it's useful to be able to see all the output next to each other at once, tabs just don't cut it. On the other hand, tiling makes less sense for the web browser or email. Hurray for configurability.
In my case since >80% of my windows are xterms it's more useful to have a tiling window manager that floats a few windows, than a floating window manager. But theoretically if a standard window manager had the option to enable tiling for a few applications you'd make 90% of tiling window manager users happy.
Judging by the comments on this thread though, I figure that's never going to happen.
BTW, there are tiling window managers for Windows, see the wikipedia article.