Rough approximations have been possible since the early 2000s, but they’re exactly that: rough approximations. Details matter, and when I boot up an old XP/7 box there are aspects in which they feel more polished and… I don’t know, finished? Complete? Compared to even the big popular DEs like KDE.
Building a DE explicitly as a clone of a specific fixed environment would also do wonders to prevent feature creep and encourage focus on fixing bugs and optimization instead of bells and whistles, which is something that modern software across the board could use an Everest sized helping of.