Multiply that code base size by like, 78.3, and you’re possibly in the same galaxy as the all the c++ codebases out there that will be maintained for the next 50 years.
Rust may eat the lunch of c++ moving forward, the language will never go away.
Just like COBOL! Seriously, _just like COBOL_. The language will fade in importance over time. C++ will be relegated to increasingly narrow niches. It will eventually only be found in "legacy" systems that no one wants to spend to rewrite. No one young will bother at all to learn C++ at all. Then, in a few decades, they'll be pulling folks like you and me out of retirement to maintain old systems for big bucks.
If the new language has good interop with the old then this doesn’t have to be painful either. A while back, I was working on a large Objective-C code base which was slowly moving to Swift. We quickly got to a point where new code was written in Swift by default. In only a couple of years, we had a code base which used Objective-C pretty much only in the dark corners where code churns slowly.