.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.
Android still runs on a monolithic kernel written in a memory-unsafe language. I'm finding it suprisingly difficult to find information on Meadow, other than it runs .NET DLLs as user-space applications, but nothing about the structure of the kernel.
Longhorn was going to be more than that. Microsoft did have Singularity/Midori projects, started around the middle of Longhorn/Vista, and continued much longer after Vista released to build out the managed microkernel concept. It's been about a decade since they've put any work into it, though.
