Build the “killer app” that the audience wants and needs and where D is the best lang to do it and justify the investment in learning yet another lang. I have no idea of what that is and nobody knows, it’s the universal problem of any product/lang/tech. Right place at the right time I guess
Of course it can, but different killer apps for a different crowd. A missile tracking system wouldn't be the kind of application to do in PHP. Wrong app, wrong crowd.
I'm aware of D since it's inception more or less but don't know it very well. I would say D lacks a "bombastic" feature and maybe that's both the reason is not used more but also why is such a good language.
It's not "memory safe" like Rust, yes it's fast but so is C/C++, it doesn't have the "massive parallelism/ always-on" robustness like Erlang. It has a bit of everything which is good and bad.
Being a mid all-arounder is OK in my book, perhaps it's more a matter of some "post-AI" tech startup adopt it and get massive or famous, like Ruby because of the Web 2.0 era or Erlang with the Whatsapp thing.
Maybe D is good the way it is and will always be there.
D doesn't have a bombastic or killer feature. What it has is elegance. It simplifies things, and smooths out the ugly stuff. You don't have to worry if your char is signed or unsigned, or how many bits are in an int, or whether to use dot or arrow, or remember to make your destructor virtual, and on and on.
It's a more memory safe language than C/C++, no need to worry about forward references, strong encapsulation, simple modules, and so on.
And let's face it - the C preprocessor is an abomination in modern languages, why does it persist?
