So let's separate out "effective" from "efficient". I'm perfectly happy to accept that the "every single card is hard" is the most efficient system for memoriz

So let's separate out "effective" from "efficient". I'm perfectly happy to accept that the "every single card is hard" is the most efficient system for memorizing facts: i.e., that if you measure the number of facts you can recite and divide it by the amount spent studying, that SRS will come out on top. But is that the most "effective" -- will it actually result in you learning more facts at the end of some time frame? For that you need to know not only the amount learned per unit of studying, but the amount that you actually study; and the amount you study depends in part on your motivation; and your motivation depends on how hard / engaging the study is. Suppose that with every card being hard, you study on average 20 cards a day; but that with every card being only moderate, you average 60 cards a day. Even if the "effectiveness" of moderate card study is only one half of difficult card study, you still end up learning more, because you've studied three times as much. The idea that after seeing a given card 30 times over the course of a year, you'd somehow end up knowing it less well than if you'd seen it only 10 times, because it wasn't "hard enough" when you did see it, seems really unlikely to me.