Adopting something like Haskell instead of Ruby isn't just an exercise for magpies who like new shiny things (In fact, Haskell is older than Ruby, and many concepts in functional programming are far older than most OO design patterns). It's embracing a different way to think about programming which can have fundamental improvements to the maintainability of the code you write, rather than just cursory ones.
No one is saying that a business problem can't be solved, or money can't be made, using Ruby.
Sure, rewriting a project from one language to another is very expensive and usually not worth it, but does that mean that we shouldn't explore other tools at all? At some point, a new project will begin, and it'll be valuable to have a more robust decision about which tools to use than just "Rails worked ok for us last time."