Why would you assume that D or Rust would be make developers more productive? If the developer already knows C++, then switching to D or Rust would make the developer significantly less productive over six months to a year, after which, the developer would likely be just as productive as he/she was when writing C++.

Languages are not panaceas. Rust and D are both hard languages, as is C++. Selecting one of these only makes sense if the application warrants it. Each have advantages and disadvantages. I don't really think any of them are suited to web development, but claiming that D or Rust would be a more productive web development language than C++ seems equally silly to seriously considering C++ as a web development language.