[WorldMaker]

I think that speaks to the big reason I think MDI fell by the wayside: the OS took on a more active role in window management. MDI makes sense in Windows 3.1 when your application can easily provide the exact same management tools as Windows itself: the classic Tile/Cascade menu items and not really all that much more. The steady progression in the power of the taskbar starting in Windows 95 I think corresponds somewhat directly in the drop off of MDI applications, as it started to get harder and harder to mimic the taskbar in your own application and Windows at least never offered much of an out-of-the-box application framework for that. Part of that was philosophical; Microsoft didn't provide a strong MDI framework because they didn't see a need (and probably felt MDI was too confusing users). (Mac OS did provide more options over the years and there are still MDI-ish applications there, so it is interesting to compare the philosophical divergences over the years.)

"Sets" are interesting here because it is a return, the long way around to MDI in a way, but with years of learning/experience of task management from modern taskbars, IE/Edge tabs, and other thoughts on task switching. I like the idea that in Microsoft's opinions Sets are more useful as diverse collections of heterogeneous applications (a Word document and its OneNote notebook and its Edge browser research tabs together in one window) rather than the classic MDI homogeneous approach. I think that may be the welcome modern twist the MDI concept was missing originally.