While I agree that it's possible to write maintainable in kdb+, I'd argue that it takes noticeably more effort; or, as in inverse, if you drop your coding standards things get worse faster. I'd say the same for regex as well (though in a more limited sense).

The "characters to clever/unmaintainable" ratios you achieve with k (which most kdb+ platforms end up reaching for at some point) are almost unparalleled. A famous example of how awesome/powerful/ridiculous k can be is the "4 lines of K" text editor: http://www.kparc.com/$/edit.k

I guess my point is that letting your guard down, for even a single line, can be orders of magnitude more trouble than it would be in most languages and for that reason I wouldn't base a new stack on it. Also, the developers for it are rare-ish and expensive + it has a serious learning curve for those unfamiliar with FP or Lisp.