The only reason C++ has been around for this long is because CPU's really haven't evolved (just gotten smaller and faster) since the late 70's
It may be a reason, but it's certainly not the only one.
But we're hitting the wall now, and we need to find new and imaginative ways to get more computational power, and in the long run, C++ isn't going to cut it. C++ is the new assembly.
I'm sure it will be obsolete eventually, I just don't see that happening soon, at least not for game engine development. I think C++ will remain at the core of many high performance engines for a while. Maybe in 4 or 5 years.
C++ is not faster by default
True, no language is faster by default, but some are inherently more compatible with CPU-friendly optimizations. The fact that Visual C++ exceeds the speed of almost any other language/compiler (including C) proves this. But the fact that Visual C# occasionally exceeds the speed of some slow C++ compilers like GCC also proves that C# might have a fighting chance to compete against C++ as a high performance language in the future (possibly), but I'll wait until the time comes before making assumptions and staking too much time and effort on it.
I'm hopeful that the "easier" languages like C# will win out over C++ over time, but doubtful.