That's a cute fairy tale from econ-101, but anti-monopoly regulations (at least in the US) really haven't caught up to the 21st century. Games (and other media based on licensed IP franchises) simply aren't fungible the same way most physical goods are.
If you want a Ford Explorer and need to settle for a Chevy Traverse, you're getting a substantially similar vehicle with different branding on it. If you want Doritos and the vending machine only has Cheetos left, it might not have been your first choice but few would enjoy one and turn down the other completely. If you want to play Halo, however, the latest Mario isn't a valid substitute regardless of price, and vice-versa.