.NET vs Ruby is a whole holy war which I'm not going to get into, but there are a lot of people out there who like to get shit done, and find that .NET (and MVC) works just fine (great even) for that.
Incorrect assumption (source: developer at huge corporation on huge projects)
Point is, there was no huge shift to MVC after it was released. A ton of web projects are still made using WebForms and will continue to be. MVC will continue to be a minority framework.
I think the shift has quietly occurred because everyone wants at least a little AJAX, and everyone wants jQuery, and once you've thrown those in the mix, who wants to do that using WebForms?
Why would you assume that if they can't move to a new architecture, they'll move to a new language and OS?
Mono? http://en.wikipedia.org/wiki/Mono_(software)
Or going in a different direction with similar goals, Qt.
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.
We're currently in the process of getting ready for a migration to Sharepoint Server 2013 which will cement the company with .Net for at least the next two to three years. We still have several sites on the old 2007 platform with no hint of moving those over any time soon.
I'm wondering what you see as viable open source alternatives to the .Net platform.
Also, we have a handful of .Net developers and they've all said a majority of the work they've had in the last year did not involve MVC3 or MVC4 so I'm also wondering what the future of this platform will be.