It's not that it's unacceptable in general (although there are certainly scenarios - real-time, embedded etc - where it is). It's that a language that effectively mandates GC cannot be a true C++ replacement. So I can't in good conscience accept a "better C++" or "what C++ should have been" label.
This isn't to say that D is not a great language. It is, but I see it more as competing against Go (which is also not "a better C"), and to some extent Java, C#, Kotlin, and Swift.
The only true contender to the "better C++" label that I know of is Rust.
You can already write C++ in D, all the features are there. On top of this, you get features that C++ should have had years ago for free: Static Reflection, UFCS (Less boilerplate), Meta-Programming that isn't a hack and lots of static guarantees (Purity, GC usage etc).
The standard library could have been implemented completely without the GC, but the GC makes programming simpler so it's not worth it. Practically, this may be a problem, but it's a little unfair to compare to use this against D (only the language) given that Phobos is not standardized in the same way that the C++ Standard Library.
In a very specific way (May or may not be good in your opinion), I think D is closer to C++ than Rust is: D maintains the basic idea of C++ e.g. Syntax + semantics are still C based, the type system is less complex than that of Rust.
However, Rust has better static safety guarantees than D. D's system is still being developed, although does already offer much more than C++.
This isn't to say that D is not a great language. It is, but I see it more as competing against Go (which is also not "a better C"), and to some extent Java, C#, Kotlin, and Swift.
The only true contender to the "better C++" label that I know of is Rust.