I think "4+5" is readable because I read it as "four plus five", and that operators become un-readable when you can no longer insert a common descriptive word in their place. The problem in haskell is not the few operators in the prelude that are unfamiliar, but the multitude defined in libraries.

In fairness, there are good ways to search the documentation for those operators so at least you aren't stuck googling for line noise.