There are no studies for anything, and if there were, they would be outdated by the time they are published.

So if you ask such a question, do you really expect anything more than people's firsthand experiences?

But I'll tell you my own side as well: doing anything with concurrency in any other language (yes, including Go and Rust) is just much much harder in comparison to Scala.

No wait, that's not true. There are languages that are better suited for that. But they are either academic or extremely niche and come with other problems (such as lack of ecosystem or much worse performance).

> The fact is there a fewer FP devs. Why is that if FP is "better"?

The fact is also that the number of FP devs has been and is growing. And so has the number of languages with those features.

> There's fewer Scala devs, because Scala isn't better.

What metrics are you using to make the claim that it "isn't better"?

Fewer devs, fewer libs, fewer tools, fewer projects.

Those are all quantified with out a research study.