I have no idea where you get the claim from. I know many members of the C++ committee, and they expect new C++ to continue to be written, else why would they bother with C++11/14/17?
Could you please clarify? Do you mean that more than 50% of the eventual total lines of C++ have already been written as of today (Peak C++, anyone)?
Or do you mean that a very large percentage of eventual lines of C++ to be written in the future are expected to be endless variations and downright repetitions of the total C++ lines been written as of today?