There are so many frameworks one can pick up and make a Windows app with, which is not bad. One of the biggest contributors to the current state of Windows is ironically the versatility. And it seems that just doesn't happen internally at Microsoft. The idea that Microsoft could implement a complete OS redesign, which includes all the in-box apps is essentially impossible given how that would require collaboration between all the different teams that work on Windows. Microsoft is a mammoth that cannot move that fast. It's certainly not out of the ordinary for Apple they did the same with iOS 7 back in 2013.
The fact that Apple can show up with an entire cosmetic redesign that not only updates shell elements but all the in-box apps, too, all in just one release is insanity to me as a Windows user. Microsoft needs to figure out a way to modernize Windows 10 cosmetically because other platforms are putting it to shame. What's more, Windows 10X isn't coming to Windows 10, so all that work means nothing to the billion users who are running Windows 10 today. But the apps and their inconsistent adaptation of Fluent Design is still a problem on Windows 10X. Windows 10X is a significant step forward in the journey to creating a consistent, clean, minimalist, and modern UI for Windows 10. This creates an inconsistent UI across the different apps and shell elements found in Windows 10 and just makes everything feel disjointed. Since Windows is split up into so many teams, a lot of those teams just don't adopt it, and those that do, have their own spin on what Fluent Design is. The other key issue is that there's no universal push to adopt Fluent Design across all the teams that work on Windows or apps for Windows.