The fallback you described would be a change in the behavior of some working programs. Moreover, it would only affect custom classes where adding a decorator is an option (even in external code, you can use `MyThing = total_divmod(library.MyThing)` under this option.
Showing that a recipe for a decorator is of wide use feels like a necessary step towards any future semantic change in the language to me.
For example, '%' is fairly widely (ab)used for meanings other than modulo. E.g. string formatting. Probably not that many classes that respond to '%' to do something non-modulo simultaneously implement `.__divmod__()` ... but maybe some use case is not obvious to me. If those exist, your change would break that (well, depending whether methods of ancestors are used or not).