Business logic is pure and stateless. Lifting to IO is a breaking change that may or may not require non-trivial refactoring. You shouldn't be putting business logic in your HTTP handlers.

Regardless, the point is not that it's impossible to make such a change in a Haskell code base, just that it may or may not require non-trivial breaking changes, while such a change in an OO enterprise code base will always be trivial.

There's lots and lots wrong with OO patterns, but being able to respond to changing and arbitrary whacky business requirements is a clear strength.