I and many many others would never have switched to Linux without Mono.
Mono lets us focus on the task at hand (develop software for a living) and then make it usable on Linux without the huge cumbersomeness and bad polish that affects many of its applications / tools and IDEs and that relegated Linux into a minority niche for desktop users.

With Mono I can develop my stuff on my workstation with all the functional tools I need and then “publish” the work for Linux.

This greatly helps crack the Big Divide between the finished commercial OSes / UIs and their zillions of users vs the poorly finished (but awesome and reliable!) open source OSes like Linux.

I agree that Mono should not replace any base Linux feature (else people may as well just install Windows, ****that***** is the real “viral” risk, not some ridicolous patent fears). But it should still be available not banned like it’s Evil Incarnate.
I still want to deliver my commercial and open source apps to Linux but if I had to step back down to waste development months using C++, I’d drop Linux and only target Windows and Macs.
Who loses in the end? Linux. It’ll keep its super-scarce good UI design developers and stick to some elite niche. The recipe to lose more and more appeal.