I believe the comparison hardening was made desirable by the expected use of first-class modules to encode existential datatypes: with existential datatypes you may try to use the polymorphic comparison operators on two values of the same (existential) datatype having different internal representations. The problem didn't happen with the previous encoding of existential types using first-class polymorphism, as it implies packing the value inside closures, so the polymorphic comparison operators were not usable.