Don't get me wrong, Mono is awesome. But it's not about to be adopted by Fortune 500 companies across the world.
But, above all, the biggest problem is not technical and not Mono itself: it is Xamarin. The money decisions on big companies are made by people that need to tell to non-technical bosses and shareholders why they did what they did. And it is much easier to talk to them about Microsoft than about Xamarin.
Remember the "no one ever lost their job for buying IBM"? It works for MS, now.
To be fair though, the parent I was replying to did not define in what capacity .net would be replaced and how and taking replies as a black and white answer to a gray area question is a bit factitious (this part was not meant as a directly reply to the parent of this reply).
You are funny.
Mono? http://en.wikipedia.org/wiki/Mono_(software)
Or going in a different direction with similar goals, Qt.