The article is presenting some stuff mixed up. Had nothing to do with the language or the framework. WinFS was a database product. Over engineered and abstract.

.NET and C# were researched a lot for operating system usage (Midori, Singularity) but that was after Longhorn.

The operating system group UI toolkits was a further problem and they pivoted there dozen of times in the years. Particular for a C++ based os group.

But the death of longhorn was ultimately about the security restart of Bill Gates