Any successor to C++ will necessarily need to be built around safety from day one. Carbon did not do that - it merely hopes to put safety in at a later stage. So no, I don’t think Carbon will replace C++.

I will be very surprised if a C++ successor will be a new language. I think it will be a frontend to C++ that cleans up the syntax, has much stricter rules around UB, and can rely on C++ as its unsafe counterpart. C++ itself will never be safe - it can’t be while maintaining backwards compatibility, and even modern features (ranges, coroutines) provide a wide range of new lifetime nightmares.

A new frontend could solve a lot of that, while allowing unsafe “trust me bro” code to be written in C++.

There’s the “enable new features in a file with pragmas” idea, too, which could be promising. But I’m not convinced.