So how do you design a language that accommodates both the people who need a codebase to be stable for decades and the people who want the bleeding edge all the time, backwards compatibility be damned?
You don't. Any language that tries to do both turns into an unusable abomination like C++. Good languages are stable and the bleeding edge is just the "new thing" and not necessarily better than the old thing.
C++ doesn't try to do that. It aims to remain as backwards compatible as possible, which is what the GP is complaining about.