You can't just take 40 years of Win32 apps and add the Metro design language, touchscreen compatibility, or dark mode system-wide. WPF nowadays has a skin that imitates WinUI, so at least Microsoft is trying.
Sure, but you could have had a uniform design language for the last 3 or so frameworks.
Right, but you can make basic adjustments to the theme to fit the rest of the system better, which took them all the way until Windows 11 to realize.