This is explicitly about the product, a good game will sell with demo even more and a not good one (majority of the games nowadays) will sell the hype. You can

This is explicitly about the product, a good game will sell with demo even more and a not good one (majority of the games nowadays) will sell the hype. You can see this in practice by watching MMOs after the first month many of them will lose tons of customers because now everyone knows the game is bad and not living to the hype. Whereabouts you can see good ones such as WoW (or even Diablo 3) which will keep adding more users.