I think go does have a niche, and it is "python/ruby devs". Yes, its niche is a set of people, not a particular role. Because go is so simple for a typeless developer to learn, and has essentially no downsides and a massive upside (performance) compared to python and ruby, I see a lot of ruby and python devs upgrading to go.
I also think you are being unfair to D. D is basically just what go initially claimed to be: a new systems language. D is a fantastic replacement for C++, go doesn't really enter the world C++ lives in.
> Because go is so simple for a typeless developer to learn, and has essentially no downsides and a massive upside (performance) compared to python and ruby
I think that's a brilliant observation! It's all of the duck typing goodness with far less of the static typing overhead. It's static types "lite" for duck typing lovers.
I also think you are being unfair to D. D is basically just what go initially claimed to be: a new systems language. D is a fantastic replacement for C++, go doesn't really enter the world C++ lives in.