A major feature of modern C++, and one of the most difficult to replace, is its expressiveness and optional strictness. You don't get this with C (or most other languages). The metaprogramming and compile-time facilities for example, while complex, are indispensable for some code bases. This becomes evident when you try to port rigorous modern C++ code bases to other programming languages du jour. A surprising amount of C++ is difficult to express with equivalent behavior and performance in a similar amount of code.
I play around with new system languages by implementing parts of database engines (typically written in modern C++) in the new language. It terms of abstract code architecture, you can port a C-style database design with relative ease -- most new system languages aim to be an improved C with direct portability in mind -- but not modern C++-style database design, and the latter is unambiguously superior for performance and correctness when writing database engines. C++ will continue to see a lot of usage as long as it is difficult or impossible to write code in other languages that is functionally equivalent to C++.
I play around with new system languages by implementing parts of database engines (typically written in modern C++) in the new language. It terms of abstract code architecture, you can port a C-style database design with relative ease -- most new system languages aim to be an improved C with direct portability in mind -- but not modern C++-style database design, and the latter is unambiguously superior for performance and correctness when writing database engines. C++ will continue to see a lot of usage as long as it is difficult or impossible to write code in other languages that is functionally equivalent to C++.