People generally have pretty good spatial sensibilities, and I feel like modern OS designers seem to forget this. You feel this especially on iPadOS. Physically arranging windows allows for a much more solid multi-tasking experience, and encouraged direct manipulation of content e.g. drag and drop. Transient "palette" or "panel" windows allow for a short term buffer (think a find/replace panel). To me, this is what m…
Single window apps came in with NeXT. I heard a theory early on in the NeXT/Apple merger that it was because the window server buffering was so heavyweight on early hardware that multiple windows just made things too slow. While I personally agree with you, I think you're overestimating the desire of most users to manage multiple window clutter themselves except in very limited contexts.
NeXT sold megapixel displays as standard. It was considerably more window-heavy than most GUIs because it was only sold to power users.
Hell, even menus went in their own window!