Or there is the other alternative -- to drop these functions from Data.Map. These functions are usually not available in other languages and the implementation is essentially forced to store the size of the subtrees in the tree. That is convenient for Adams' balanced trees, but not for AVL trees, red-black trees, tries (IntMap) etc.
I don't Data.Map is supposed to be an interface, it's an implementation. I don't think there's anything wrong with a data structure providing operations you can do efficiently in that data structure, even if other data structures can't. In fact, that's often the point of having different data structures. As to making IntMap a drop-in replacement, should it be? If you want to switch to IntMap for performance but need to do some things are going to be inefficient with it, then you should probably either not switch, or come up with a different way of doing things, right?