It is entirely possible that the best developers on this planet using the best tools and methodologies, processes, whatever, are just not smart enough to do some things.
Considering how many things we have achieved, we should just acknowledged this human attribute: imperfection.
If our tools require us to be perfect, maybe they're just not good tools for some goals.
I'd be shocked if Mozilla didn't have some of the best C++ programmers on the planet, and if they failed a few times, that's a pretty strong argument against C++.
Who knows, maybe they'll be proven wrong and another team working purely in C++ will achieve goals such as theirs. I wouldn't bet on it (though C++ is evolving so who knows how C++ will look like in 20-30 years, never say never).