You're quite right about the juxtaposition of "demonstrable" and "future". I should have said functional programming has "already" demonstrated an advantage in producing robust and understandable concurrent programs, which will continue to be relevant in our multicore future. Really, there's nothing "theoretical" about that fact.

To your second point, I made no claims that C and C++ are not used to solve performance intensive problems. I'm sure C and C++ (or Assembly, for that matter) can be applied to solve any number of problems, if that's one's calling.