Functional programming is definitely worth learning. It will make you a much better developer. What I’m advocating here is to apply JEFP (Just Enough Functional Programming) to deliver working, maintainable software without forcing yourself to achieve 100% purely functional solutions.
Nothing, is the answer. There’s nothing programmatically in Ruby to stop you using its sharp knives to cut ties with reason. We enforce such good senses by convention, by nudges, and through education. Not by banning sharp knives from the kitchen and insisting everyone use spoons to slice tomatoes.