How can it be a niche language, if it's an ANSI standard, has more than 8 or 10 fully feature, standard-conformant implementations, runs on most CPU types, and has been proven to work in production systems for spaceship guiding, worldwide airfare reservation and credit card transaction verification?
You use Js and Python because you choose to use it, but it's not the only choice. Not only Common Lisp, you could also be working in Clojure with many benefits.
It is not. A language feature is either amenable to static analysis (not necessarily type checking) or it is not.
> Consider dynamic class loading in Java, dlopen / reinterpret_cast in C++ etc.
This actually emphasizes my point. Features that are not amenable to static analysis are problematic for tooling.