KDB might be niche, but that niche is finance and there are a decent number of open KDB positions. I get a couple of unsolicited emails a week for them. And they pay very well.
Yes, Clojure (that Lisp variety) as well. Forth for embedded as well. But they are niche; it depends where you live etc on which opportunities there are for each of those. But yes, you are right; Kdb (and Clojure btw) are good in some parts of finance and pay well.
Edit: what i'm basically saying is that practicing any of these languages will help you in potentially more than one way; it will open your mind to interesting paradigms and constructs as well as give you job opportunities you would not think of possibly later on.
