The people who are getting mad at Activision over this one have to realize that open-world games are crazy expensive to produce, and so for an open-world game to even be considered successful, it basically has to be what would be considered a "home-run" by the standards used for other games.
Bethesda Studios spends years and years on their Elder Scrolls games, while Treyarch spends a year and a half on a CoD game. CoD selling a few million copies would still make it extremely profitable, because the development cycle is so short, and the proportionate amount of work in a linear game is much smaller. Building (and more importantly, actually rendering) an entire world takes a much bigger budget, and so selling only a million copies might not actually end up producing much of a profit.
Gearbox sold 4 million copies of Borderlands in 2009, and they still weren't sure if they were going to be making a sequel. That tells you something right there.