I'm surprised that you think FP requires mastering category theory. Au contraire. It's perfectly possible (and indeed easier for programmers) to understand the abstractions that Haskell and Scala pioneer (higher-kinded types, and things like type-classes as well as monads which are basically enabled by HKTs) without category theory. It is true that monads were discovered via category theory, but in retrospect programmers could have discovered them -- after all they are (simplifying a bit) merely a way of generalising composition.

Scala and ML-variants like Ocaml, F#, Reason.js are perfectly good imperative languages. They don't force you to go purely functional. I always recommend that my students (whose first language is Java) start by treating Scala as a "nicer Java", and then slowly explore Scala's more powerful features. Indeed, I'd argue that Scala and the aforementioned ML-variants are (slightly) better general-purpose imperative languages than the Javas, Pythons, PHPs and Javascripts of this world, because they are more consistent, they have types/type-inference, and their statefulness fully supports higher-order state and powerful modularisation without restriction, not to mention pattern matching.

Being able to use functional idioms without compromise if and when desired (including, but not restricted to, pure functional programming) is another benefit of Scala and ML-variants.