But there still may be some truth to the claim. A simple fact is that smaller mind share -> fewer programs -> less chance for extremely successful projects. From personal experience: it took me three tries and multiple months to get comfortable enough with Haskell to the point that I was able to write my first contribution to pandoc (the org-mode parser), despite having dabbled in functional-style Lisp for years before that. But Haskell, as used by pandoc, isn't difficult. In fact, I often find it easier to use Haskell, thanks to its excellent type system. It's just very different and requires a bit more investment up front, with huge benefits lurking down the road.
Data to support my claim that Haskell is actually easy to use: over 300 people have contributed to pandoc, with over 100 contributing Haskell code. Many of those contributors have never written any Haskell before, but the type system helped them to find their way.
I talked a bit about the whole topic here: https://youtu.be/JpNEIpLtCHs
