> Why would it matter and to whom if C++ use would decline?
If the use declines to zero, then all the effort someone put into developing C++ compilers and related tooling had been for naught, as are C++ development skills.
Given the prevalence of C++ it is very hard for me to imagine a situation where the use would decline to zero.
The notion of how to write a specific language such as C++ is immaterial compared to the capability to design and implement complex software systems and those skills are quite portable between languages. The best employers tend to recognize this.
If the use declines to zero, then all the effort someone put into developing C++ compilers and related tooling had been for naught, as are C++ development skills.