What C++ is mostly used for are games and desktop GUIs...
Hardly. Just about every interesting native application is written in C++. Chrome, Photoshop, Cubase, Ableton Live, AutoCad, Java, V8, any AAA video game, etc.
Whether you like C++ or not the fact remains that if you want to write an application of any complexity that really pushes the hardware it's just about your only reasonable choice. Maybe people still write kernels in C but kernels really aren't that complex compared to the applications they run.
You listed mostly desktop apps and games = exactly what I said. And even in this area, C++ is not used exclusively - see Minecraft (Java), Azureus (Java), Gimp (C), VisualStudio (new code mostly C#). So no, not every.
BTW: There are many interesting and successful complex applications really pushing the hardware that were not written in C++: Cassandra (Java), Hadoop (Java), PostgreSQL (C), Netty (Java), Nginx (C), TeX/LaTeX (uh, this one is really interesting). So, again, C++ is not the only reasonable choice when performance matters.
Scaling across multiple machines is a completely different problem than pushing a single machine to the limit, with completely different constraints. Once you move beyond a single CPU and you can tolerate higher latency then languages like Java start to become more viable.
When you need maximum performance and minimal latency from a single machine C++ is still the first choice and the number of high-performance apps written in C++ vastly outnumbers those written in C. Check high frequency trading for another example.
Hardly. Just about every interesting native application is written in C++. Chrome, Photoshop, Cubase, Ableton Live, AutoCad, Java, V8, any AAA video game, etc.
Whether you like C++ or not the fact remains that if you want to write an application of any complexity that really pushes the hardware it's just about your only reasonable choice. Maybe people still write kernels in C but kernels really aren't that complex compared to the applications they run.