First: they didn't write a runtime. They used .NET/Mono as their runtime.
Second, you wrote your comment in a text box rendered by an application notorious for being written in a custom language (in fact, it seems like arc:lisp::wasabi:.net).
Third, do you have evidence to support "nine hundred ninety-nine times out of a thousand", or is that hyperbole? Have you worked on projects where people used custom languages? Your argument would be much more interesting if we could read more about your experience with it.
Second, you wrote your comment in a text box rendered by an application notorious for being written in a custom language (in fact, it seems like arc:lisp::wasabi:.net).
Third, do you have evidence to support "nine hundred ninety-nine times out of a thousand", or is that hyperbole? Have you worked on projects where people used custom languages? Your argument would be much more interesting if we could read more about your experience with it.