Earlier quoted context omitted.
Mono is an option for the current generation to learn one less language. Outside of Windows it's not the default language for anything except Unity3d so curious kids who grow up with tablets might not have much reason to learn it.
So, nobody should learn a language unless it's the "default language" for a platform? What is Python, Ruby or PHP the default language of? Should anybody be learning Perl, Haskell or Objective-C in your opinion?
And yeah this isn't at all specific to C#. Anything not natively supported suffers - just ask Flash who in a handful of years have gone from a defacto standard to a dinosaur.