For C# Microsoft eventually embraced NuGet their package manager, and used it to put core packages that don't need to be fully available OOTB but can then benefit from frequent updates on a per project basis as opposed to updating the entire language runtime.
For Go it was the out of the box packages, like if I want to make a website, I can pull in net/http and their templating packages that come out of the box with Go, maybe a reasonably simple core maintainer package or packages that go into Dub would be a strong selling point. Right now Vibe.d is the only option for web dev, but there's no reason a much simpler web server couldn't exist.
For Rust, I just really love Cargo, I think its one of the nicest package managers I've ever used.
The other thing that would really help D is if something significant is built around D, whether it be a framework (like what Rails did for Ruby) or some major application that needs D to function at its core and is used by many, this could be a groundbreaking modern IDE, or anything really, a database that uses the best bits and pieces of D to scale, or even a really rich cross-platform GUI stack (my kingdom for std.gui to be a thing in D, and reasonably exhaustive).
I wish I had unlimited time and money, I would invest it in D. Alas, I'm not a language maintainer, just a guy who loves really good tools.
To the parent's point of startups, betting the farm on something like a particular language out of some sense of superiority might mean you are not focusing on the right problems. But if the founders happen to know a less widely used tool it doesn't seem inappropriate either. The type of employee that can drive a startup or a big tech project forward is not going to be thwarted by a language, and they might find something new to learn fun.
Maybe it exists and I am just ignorant but it doesn't seem to be in the list of supported languages.