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.
Well, what's it worth if you are doing (real-world) number crunching, and your program turns out to be one or two orders of magnitude slower? For instance, after implementing some machine learning software in Scala, I ended up rewriting it in C++, because it was so slow (IIRC the Scala version was 50x slower). The only way you can get down to 'a couple of times' slower is by writing Scala as Java with a slightly different syntax. And since the program does many repeated matrix calculations in a loop, parallelizing the C++ version was done in half an hour (mostly testing where lock contention kills the advantage of parallelization).
Yes, I do realize that there are more applications of multicore processing than number crunching ;).
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.