I get why they'd need to support all that for 3rd party app backwards compatibility, but surely for their own OS they can just use the current toolkit for everything? They own it after all.
Win32 is quite basic in the set of controls available, almost everything in traditional Win32 applications is custom drawn, beyond what you also find the HTML forms as components.
Then there are all commercial components that are so common in Windows applications.
However if you will, the future is WinUI/UWP, and that is the only one getting full Mica support.
