Rails has so much metaprogramming that static typing isn't really going to get you very far. Type checkers and linters won't know what the hell is going on.
ed: to the degree that the man himself is adamantly against them in his language (except as perf hints) https://bugs.ruby-lang.org/issues/9999#note-13
See also mypy's recent blog post on plugins (which are also used for supporting Python 3.7's dataclasses, which are quite dynamic as well): http://mypy-lang.blogspot.com/2019/03/extending-mypy-with-pl...
If I compare that experience to when I switched from JS to TypeScript, it's like night and day. TypeScript is designed to work well with development tools and already existing libraries, while Python's static types feels designed by someone who never worked with static types while completely ignoring code patterns used in existing libraries.