> Until Stroustrup fixes the header processing behavior (it's 2019 already...) C++ will not become a sane language to use for new development.

C++ does not have a BDFL like other languages, C++ evolution is governed by a committee. In other words, Stroustrup himself needs to submit any changes he wants to the committee and there is a vote ...

C++20 modules should partially solve the header processing problem, but it will take years until it will become widespread in big companies.