D didn't replace C++. I wonder if that was related to the single proprietary implementation. The language seems more sensible to me.
Rust is aimed at replacing C and C++. There are others in the same field in various stages of development. Zig and carbon come to mind.
There's an uphill battle where lots of existing code is in C++ and people fear changing it. C was partly dismantled by C++ compilers promising better error diagnostics and being able to compile most C with minimal patching.
I personally think the end is in sight for C++ but the consensus seems to be it'll live forever.
But Rust brings you OOP-like structures without the C++ hidden machinery, generics (the two main reason people adopted C++), and a lot of code verification. And the language isn't much more complex than C. That combination is very enticing.
I do believe both languages are about to die (C and C++). It's just that when languages die, nobody notices for a decade or so. Even develovers keep getting more numerous for some time, and have their numbers slowly reduce through decades.
As the office Rust shill, I'm actually pretty excited about a lot of Herb Sutter's work [1]; I think there's a reasonable argument to be made that a hypothetical C++29 will be as ergonomic as unsafe Rust, with the not inconsiderable benefit of being able to speak the C++ ABI. I don't think large projects like LLVM or Unreal would dream of rewriting to Rust in that time, so people will still be writing it in some form or another.
[1]: My top 3 would probably be cppfront's argument passing semantics, metaclasses, and deferred_heap, in that order. The latter two, IMO, are improvements on what Rust provides in the area.
D wasn't Boost licensed when I looked at it as a replacement for C++ about a decade ago. Wiki thinks the backend has been available since 2017. It looks like the source may have been available for longer than that but I missed the distinction between source available and open source if it was. That's how I ended up on the C++ path instead of the D one anyway, seems a credible risk others have the same experience.
We couldn't get the dmd backend open sourced until 2017, but everything else was open sourced.