[ashmud]

(Not directly addressing what you're saying.) A few MMOs have been written using .NET.

https://gamedev.stackexchange.com/questions/10247/mmos-based...

[AdeptusAquinas]

As well as a tonne of games in general, which I'd argue are 'hard problems' as well. Pillars of Eternity, Endless Legend etc. Though of course none of those are OSS. Would be nice if at least parts of them were as it would certainly help indie game devs like me :)

[zamalek]

In addition quite a few MMO server "emulators" are written in C# (e.g. Mangos, WGrid).