The length of a game varies according to what the players do. If your opponent plays a slow strategy, and you a quick one, not only will your deck become useles

The length of a game varies according to what the players do. If your opponent plays a slow strategy, and you a quick one, not only will your deck become useless to do anything with, but you will lose your power to control game length (as it is tied to buying cards), so your opponent will be free to extend the game length.