I read that those were literally a one week thing, most new products (for over superstars, and good products) will out sell Cena for a week or two then drop back to the typical 5th, 6th places. Punk was on top for months. It's like Xbox games. MW3 for example will always be a top 20 game, but when gears of war, or fifa, or a new game comes out it'll out sell it until it stops being hot and it'll dip under mw3 again. Summerslam drew poorly because of how shit the story-line went. Punk returning instantly in a one vs one against Cena, it was quite obvious who was going over at Summerslam, and it was obvious Del Rio was cashing. Though I suppose the argument is about casuals.
MITB was when the story-line and his push was at his prime, Summerslam was the end. I don't think any one can deny that they should have kept his previous character in comparison to this new vanilla midget Cena.