> I take a 'living' language any day over of a language that's ossified because of strict backward compatibility requirements

Maybe you would, but >95% of serious projects wouldn't. The typical lifetime of a codebase intended for a lasting application is over 15 or 20 years (in industrial control or aerospace, where low-level languages are commonly used, codebases typically last for over 30 years), and while such changes are manageable early on, they become less so over time.

You say "strict" as if it were out of some kind of stubborn princple, where in fact backward compatibility is one of the things people who write "serious" software want most. Backward compatibility is so popular that at some point it's hard to find any feature that is in high-enough demand to justify breaking it. Even in established languages there's always a group of people who want somethng badly enough they don't mind breaking compatibility for it, but they're almost always a rather small minority. Furthermore, a good record of preserving compatibility in the past makes a language more attractive even for greenfield projects written by people who care about backward compatibility, who, in "serious" software, make up the majority. When you pick a language for such a project, the expectation of how the language will evolve over the next 20 years is a major concern on day one (a startup might not care, but most such software is not written by startups).