> They did not do a lot of mistakes in the 25+ years

If my memory serves, .NET and WinFS were the two major forces that sunk Longhorn, and both have been given their walking papers after the reset [1].

.NET and C# have grown to be mature and well-engineered projects, but the road there was certainly not without bumps. It's just that a lot of the bad parts haven't spilled outside of Microsoft, thankfully.

[1] https://www.theregister.com/2005/05/26/dotnet_longhorn/

Are we mixing the language and the runtime here? C# the language seems weirdly free of weirdness and footguns.

.NET was already a going concern before Longhorn even started. What sank Longhorn was the fact that writing an OS from scratch is hard and maintaining compatibility with existing OSes in the process is even harder, especially when you're adopting a completely new architecture. Longhorn would have been a microkernel running 100% on the .NET runtime, mainline Windows is a monolithic kernel written in C++. I don't know how it would have ever worked, whether .NET was "perfect" or not.