Nice theory, but completely false.
tl;dr C++ lets you do stuff the alternatives don't, it lets you directly leverage existing infrastructure, and it lets you pay only for language features you use. No alternative does all that.
The whole point of object files is that you can call the functions from ANY language. You can call compiled Perl functions from C. You can call compiled Java functions from C#. You can call compiled C++ functions from Python. Etc, etc..
The only thing you need to know is the function's interface. Tools like SWIG can automatically convert them for all major languages. Other sites like pinvoke.net are specialized to C# or similar tools for JNI. Before the tools you had to do it manually but it was not a difficult task.
The barriers between languages has not existed for decades (since the mid 70's) when languages started compiling to common object file intermediate formats.
: Add pinvoke.net and JNI tools.