My point is that there is nothing about functional programming, specifically the "Data structure + Algorithm" style, that makes it unsuitable for building complex applications like air traffic control system or bank lending system, and it has in fact been used successfully in domains like that.
If you are really interesting in this topic, check out the following pages: https://clojure.org/community/success_stories
https://wiki.haskell.org/Haskell_in_industry