That's a good point - it is highly dependent on what languages you already know from beforehand. If you know JavaScript already then it makes sense to learn a language that is not garbage collected, has no mutations or is not managed.
I would pick up Rust or Clojure before Java in this regard for example - though learning new languages gets way easier the more you learn.
"Given the difficulty/time requirements of learning something and the salary/technology trends of our industry, does it make sense to invest in the mastery of abstraction X?"
The time I have outside of my sprint is really limited, so getting expert guidance on whether knowing Rust will benefit me more than knowing Java is in fact extremely valuable to me.