I don't see how this has any bearing on anything. Should Mirage be in its own special category outside the other sets because it was designed to replace a core set and the others weren't? You already said that. Care to repeat it again? Yeah, but that's because it's so old that it predates the modern block system. Ice Age does too. It being part of a three-set block with Alliances and Cold Snap was retconned. Mirage is arguably the same way, but I don't know what all of the designers' intentions were there. In any case, extrapolating from what you're saying, I see no reason why Ice Age should be in the same category as the other large sets any more than Legends. Mirage is questionable too. As for staple common cards, do later sets not count because they don't have Disenchant? Legends has its own "staple" commons. Most of them are bad, but they do exist.