I wonder if our current state is a consequence of the 'iteration' of skeuomorphism, where a few decades ago computer UI controls were representations of what people would have been familiar with before - physical buttons on machines or concepts like files in folders, more recently the feedback loop is abstract controls being the input for the next iteration. How many objects in the world that you use in a week give a good template to inspire designers?
Whether MDI was good or not, eh, probably not. But I never would've expected that (of all things) to trigger nostalgia for me.
People have made versions for XP, 7, etc. as well.
Mac OS 9 did this well, but that was because it was effectively single tasking in terms of the user experience.
When portable CD players were still chunky companies would give them a bit more funk, buttons that followed the curves of the device and such. Similarly, before MP3 players went tiny there was some extra space for a bit more design that some Winamp skins would mimic.
In contrast, a lot of classic 1990-2010 stuff was stable and boring because it had reached a point of working well and the visual indicators were consistent.
Now it's more like "pandering to the lowest common denominator of a touchscreen interface, and doing it badly", or "sacrificing good UX in the name of looking different".