Even if a case could be made that people only use one window at a time, they do switch windows, and one of the quickest/easiest ways to switch windows (or modes) is to click on another window, which is only possible isif the other window is (at least partially) visible. The overlapping-mouse-selectable-windows model is so useful, versatile and easy to learn that it's survived for 40 years. And beyond the rapid task switching that's possible, the overlapping windows is somewhat metaphorical to papers on a desk and the sense of location (word processer here and web browser there) is comfortable and efficient, and arguably intuitive.
Many of us have worked with pre-GUI systems, having to switch between thean editor (taking up the whole screen) and the command line screen (taking up the whole screen) and other things (taking up the whole screen), so the one-screen-at-a-time approach has been tried and thought Windows 8's return towards that model, whenif not constrained by hardware issues (i.erejected, relegated to niche use. tablets Multiple non-overlapping windows, and phonesmany other windowing models have also been tried with small screens) was a major mistakelimited success. Today's common overlapping windows model has survived, flourished, and dominated.
Microsoft will definitely be rethinking their one-screen-at-a-time model.