Ever heard of updates? Also, that's what "buying" a digital game is: supporting the developer. You're not buying the bits, those are infinite. It's really a gift exchange.
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I am saying, is if the game is finished and there is nothing else being added or changed then there is no need for a monthly service. Like an MMORPG. Buying the game is like buying a product. A monthly fee is more like renting it. In the end you pay way more then what the game is worth if you could have just bought it.