“Program Manager wasn’t broken” doesn’t really contradict “don’t fix what isn’t broken.”
The point isn’t that interfaces should never change. It’s that changes should solve identifiable problems. Program Manager eventually stopped scaling well to how people were actually using computers, and the taskbar/start menu ux we all use today solved most of those problems. It took iterations, but we've got something pretty solid these days as compared to Windows 3.11.
If you think the current desktop UX has equivalent problems, identify them. “We’ve used this paradigm for a long time” is not itself a reason to replace it.