You can't reduce it to mutual exclusivity. Their plan is logically possible; it just isn't likely by a long shot. Unfortunately, it is practically impossible to explain the economic studies that go against the Ryan plan in a short debate format (and probably impossible to explain them to the vast majority of the American populace in any case). So I can understand why Obama is simplifying it to an either-or proposition. Sometimes the best you can do is teach the lie that leads to the truth. But it still grates.