It's a massive, massive risk there's nothing certain about this game selling as much as they need it to do. It may do it, but we've seen countless studios attempt to do it and fail.
The market is so easily predictable that you can invest this kind of money and expect all to go well. And if this fails, then your company is in some serious, serious trouble.
If you're this wary, how do you think the people actually in a position to be affected feel? I think it's odd how many people in this thread have expressed such sentiments with the pretense that everyone at Activision is too stupid/oblivious/whatever to see their point.
It's indeed a big risk. But the potential reward could be much bigger, and Activision has an enormous buffer either way.