WinForms' key is the quick drag and drop and edit properties type designer. It's not really intended for you to restyle it entirely (though you can). In that it was 'the style' of apps of the day, you could simply put whatever elements you needed on a form, and then double click on them to put some code behind them.
Since WinForms' existing styling was the style of applications of the day, there was little reason to heavily restyle it. Honestly, if someone ran with the new open source WinForms and introduced a Windows 10 style that I could build with the same incredibly easy editor as the existing WinForms, I'd be set.
There was something to that style. Different application UIs were pretty predictable in use compared to what we have today. I think there is way too much work going into messing with the UI instead of real functionality.
Since WinForms' existing styling was the style of applications of the day, there was little reason to heavily restyle it. Honestly, if someone ran with the new open source WinForms and introduced a Windows 10 style that I could build with the same incredibly easy editor as the existing WinForms, I'd be set.