I agree that this can be nice when done right (Clojure), but null is a high price to pay for this convenience.

I must admit I’ve never had this problem in application development. In fact, I do want to change my callers because strengthening the contract is an opportunity to simplify the callsites - they no longer have to handle the optionality. The change might carry some semantic meaning too, why are you getting x instead of Maybe x all of the sudden? Are there some other things you should reconsider in the callers? I can see how it could be useful in library development, but there are also patterns to account for this that are idiomatic to Haskell.