Is the idea that you would take your existing code and add typesafety to it? A statically typed language is faster because it does type checking at compile time and does not need to do "if string, ok else throw exception", so adding a bunch of type checking to a dynamic language adds the development overhead without the benefit. I suppose there is a false economy in that statement as defects caught early save time vs late defect discovery, which in my experience, is very common due to type issues in dynamically typed languages.

I personally prefer working with statically typed languages but I've been working with elixir for the last 8 months or so and do not feel very compelled by the use of dialyze (type checker) in the dynamically typed language - you get used to the idioms and try to keep the code type-y but it seems unidiomatic to take type safety too far in a dynamically typed language when you don't get the performance benefits. You cover the things you're likely to get wrong but embrace it for the sake of development speed otherwise. For a new project the right question to ask is "why would we use this instead of a statically typed language." I suppose for an existing project the question this is trying to solve is "how can we reduce bugs now that we've made it to market."