Why? What exactly would keep a second hand digital games market afloat? Physical games have collectability. You might pay a little extra to buy new, so you know the physical goods are in pristine condition. Digital goods have no inherent value. You can show them off on your Steam account and that's about it.

People would buy the keys at initial lauch, finish the game and then sell the keys. Next group buys those keys for cheap, finish the game and then sells for even less. This cycle continues in a race to the bottom. Unlike physical media where it could get lost, destroyed, etc. those keys NEVER go away. Prices will go down infinitely. There is absolutely no scarcity whatsoever.

Companies are only able to sell a certain amount of keys total before the third market economy kicks off and everyone just uses that. Companies then have to maintain price parity with the third market and sell their games at perpetually low prices because there is NO downside to buying used in a digital market. Aint no way in hell a company is sinking money into big-budget single-player games if they have to sell the game for $5 a month after release. They would need to shift towards making more replayable games to incentivise people to hang on to their copies.

Please, tell me where I am wrong.