Indeed. I would argue that a better line for Haskell would be "What if everything was a value?" Basically the IO monad is a way to construct imperative programs

Indeed. I would argue that a better line for Haskell would be "What if everything was a value?" Basically the IO monad is a way to construct imperative programs by composing side-effect free values. And indeed, much of the subsequent work in Haskell has been materialising even more details to avoid the hidden side-effects that cause the IO monad abstraction to break down.