Depending on what you consider the runtime, a lot of it is already written in C#. And many new concepts are written in C# when they may have been written with C++ in the past, in part due to C# supporting more low-level concepts than before.
That said, large-scale rewrites don't happen just because engineers feel like doing them. In the case of Roslyn, there were multiple compilers (C# language compiler, VB language compiler, C# tooling compiler, VB tooling compiler, etc.), requiring an enormous cost to evolve C# and VB as languages while ensuring that end users of these languages had a good experience using tools like Visual Studio. Beyond that, there was a host of additional tooling to provide more advanced code analysis that was equally expensive to maintain and evolve as the languages evolved. This meant that the kinds of language and tooling innovations end users expected was more challenging to meet. A similar set of challenges - focused on problems centered around the end users - must exist to justify the enormous cost of rewriting a massive engineering system.
That said, large-scale rewrites don't happen just because engineers feel like doing them. In the case of Roslyn, there were multiple compilers (C# language compiler, VB language compiler, C# tooling compiler, VB tooling compiler, etc.), requiring an enormous cost to evolve C# and VB as languages while ensuring that end users of these languages had a good experience using tools like Visual Studio. Beyond that, there was a host of additional tooling to provide more advanced code analysis that was equally expensive to maintain and evolve as the languages evolved. This meant that the kinds of language and tooling innovations end users expected was more challenging to meet. A similar set of challenges - focused on problems centered around the end users - must exist to justify the enormous cost of rewriting a massive engineering system.