The real problem with C++ is not slowness, but being too complex and unpredictable. I think that what will happen is that C will get the few good features from C++, and the rest will die.
You're correct that C++ is typically no slower than C, but it seems very unlikely to disappear anytime soon. There are probably billions of lines of C++ out in the real world. It will never be the most popular language, but it's a very significant one, and will be for quite some time. C++ is used when you need the performance of a to-the-metal compiled language like C, but need better abstraction models for large, complex systems. But unlike some other languages, you typically pay little to nothing extra for these abstractions, as the burden is shifted to the compiler. There are many times when performance really does matter, and you can't simply afford to throw more hardware at a problem, such as a very complex application on a single client PC, a videogame console, or at massive scales like in mega data centers.
If you think C++ is "unpredictable" then you just don't know the language all that well. I'm not trying to sound arrogant or condescending, as it's absolutely a difficult language to learn and especially difficult to master (hell, probably near impossible to master it *all*), but "unpredictable" is how you describe managed memory, not C++. Yes, C++ has a lot of sharp edges as a language. It's ugly, clunky, bloated (aka "feature-rich"), slow to compile, and difficult to master. But it also has a large, mature ecosystem (thanks to its C-based heritage), and damn near every significant CPU or platform has a C++ compiler that supports it, probably topped only by C.
So really, it's reasonably safe to say that neither C nor C++ are going anywhere anytime soon, thanks partially due to sheer inertia caused their pervasiveness through our critical infrastructure. This fact alone dictates that compiler support will remain a priority among major software companies (Microsoft, Intel, Apple) or projects (Clang, CGG). Add to that the enormous codebases that companies have invested in with both languages, and C/C++'s longevity is even more likely.