Perhaps you should compare and contrast a similar conversion...
Ogre (C++) to Axiom (C#). Note that Ogre/Axiom essentially only deals with 3-dimensional rendering and not other aspects of an ''engine''. Also of note would be the relative age of the projects and amount of work done on each, Ogre has been around much longer and has many more developers, Axiom has the benefit of faster productivity (I''d, guesstimate that given the same C++/C# profeciency you''d likely develop 20%+ faster in C#), but also it has Ogre as a template to work against, obviously saving much design and implementation time, though that would be in line with your current situation.

There''s a lot of FUD and not enough concrete data. Try it out and see. I will say that if you''re developing on Windows, .NET is not going to go away, however, what you will see is a reduction in the amount of unmanaged code. Weigh the pros and cons, try it out, measure it, and see if it meets your needs and goals. Of course going to .NET doesn''t mean that you have to go to C#, you could go with managed C++ or any other .NET language.