These folks are really onto something, but I think they get sidetracked in the (admittedly very very fun) minutia of the languages and lose sight of the crucial insight in re: mathematical notation, to wit: it's a means of human communication.
For APL or K to get out of their niches would require, I am convinced, something like a tome of documentation of a ratio of about 1.5 paragraphs per line of code. That would give us mere mortals a fighting chance at grokking these tools.
A similar problem plagues the higher-order stuff they're pursuing over in Haskell land. I know e.g. "Functional programming with bananas, lenses, envelopes and barbed wire" and "Compiling to Categories" are really important and useful, but I can't actually use them unless some brave Prometheus scales Olympus and returns with the fire.
Stuff dribbles out eventually. Type inference and checking have finally made it into the mainstream after how many decades?