That's too optimistic. C++ would easily die in 20 years if it didn't already have 30+ years of still-active legacy that can't easily be converted or rewritten.

I've recently even had to start new projects in C++ because platforms I depend on demand it or because I have to interface with existing code and libraries that still only exist as C++. I'm not a fan of the language by any means, but I'll eat my shoe if it's "dead" in 20 years for anything except maybe greenfield development.