I think the reason why Scala is often used for these niche applications is that:
A) The language doesn't fall apart as soon as you try something not imagined by the original language creators.
B) Pushing the boundaries actually returns useful results.
C) Java isn't able to solve the given task or at least not productively.
That doesn't make Scala a non-general-purpose language.
Probably the majority out there is using Scala as a better Java and is perfectly happy with it.
Comments
I think the reason why Scala is often used for these niche applications is that:
A) The language doesn't fall apart as soon as you try something not imagined by the original language creators.
B) Pushing the boundaries actually returns useful results.
C) Java isn't able to solve the given task or at least not productively.
That doesn't make Scala a non-general-purpose language. Probably the majority out there is using Scala as a better Java and is perfectly happy with it.