This kind of attitude towards less popular programming languages is very damaging to the field of software engineering. Domain-specific languages are our best tool to combat the excessive complexity of software. The difficulty of learning a new programming language is overblown.
When people say they resent having to learn another language, I wonder if it's really the language they don't want to learn or another set of libraries and frameworks. I mean, that's fair enough, the wheel only wants reinventing so many times.
Windows has Windows Forms, WPF, and UWP; macOS has Cocoa; and Linux has … well, a lot of things, each with their own quirks, and not all of them with bindings into every language a developer might want to use.
Steve McConnell observed something like this in "Code Complete": an experienced programmer can get some level of proficiency in a new language in a few weeks, but mastering the tools, libraries and idioms of a language can take a couple of years. Hence, it's a pretty big investment for a whole professional team to switch platforms, and you should try to avoid it.
OP argued that the choice to go with a niche language limits contributions, which is almost certainly true, even if it does combat complexity. You can both be right.
I think once you know 2 (or 3 if the 2 are as similar as, say, Java and C#), picking up new ones is comparatively easy. New paradigms can be challenging (looking at you, Prolog and Haskell) but quite doable.
However, if you resent being forced to learn a new language, or if you are more of a copy/paste developer than someone who really understands what you're doing[0], yeah, a new language can be a very challenging barrier.
[0] I want to be clear: most people start out with very little clue what they're doing in programming. That isn't meant as a disparaging description. I wrote (and copy/pasted) a hell of a lot of code before I started understanding the underlying mechanics of it all, and I'm still baffled by a lot of the development world (again, hi Prolog and Haskell).
>The difficulty of learning a new programming language is overblown.
I'm not sure that's the primary reason for people to disagree. A larger reason is that a smaller language has a smaller ecosystem, and solutions in a common language need to be completely rewritten to the new language.
