Many people will not buy Ace of Spades. It's not right to release a game for free for about a year or two, then make people pay for it at the height of it's time. That's going to cut your traffic down, by a lot. Minecraft's traffic never really took a dramatic decrease, for as far as I know it was always being paid for. Maybe it was being paid for less at the beginning, but that was when it was in Alpha and Beta form. As the features were unlocked, the price was risen.
I could see Ace of Spades being paid for, just not in the sense of the whole game. If we had to pay for "premium" guns, airstrikes, and other extras, like maybe boats, then it would turn into more of a success than a (predicted) downfall. Ben can turn this game into a profit if he did it wisely, but I don't think that making people pay for an originally free game is a smart idea at all.