That while a new keyboard technology like touch surfaces that allows people to have the APL glyphs at their disposal will help increase adoption a little, APL will always remain a niche language.
It's not the "being able to type in the glyphs" that hurts it, as much as the reading them -- and the understanding of its concepts.
as I say below I agreed with the change of interface impacting user perception of apl.
I use a HP48 pocket calculator, that comes with Lisp/Forth (RPL to name it) system. The interface is live interaction with a stack and a bunch of direct screen shortcuts.
I felt it was almost as fun as using emacs with a good lisp configuration. All this with a handful of keys and one level deep keyword folders. I'd bet a dollar that the same thing with an APL system would make people enjoy the language right away.
"I've a theory that APL might become increasingly attractive as touchscreen computing matures (part of the theory is that touchscreen computing is not mature)."
At best it might become increasingly MARGINALLY more attractive.