Unfortunately, I don't think D is good enough to prove your point. From your follow-up comment:

A language that for all intents and purposes is irrelevant despite being exactly what everyone wanted,

As someone who uses D, I can attest that it is not what everyone wanted; at least not yet. Despite all the great things in the language, the ergonomics around actually using it are mediocre at best: Several of its appealing features quickly turn it into a noisy language, error messages are often so obtuse as to be useless (especially with templates and contracts in play), and Phobos (the standard library) is practically made of paper cuts. Also, the only notable async support is a fragile mess, and garbage collection is too deeply embedded into both the stdlib and the ecosystem.

(To be fair, D could be vastly improved with better defaults and standard library. That might happen in time, as Walter and the other maintainers have shown interest, but it's just wishful thinking for now.)

Also, D is an entirely different language from C++, and as such, would require code rewrites in order to bring safety to existing projects. It's not really comparable to a C++ extension.