hedwards: [1]How many people are going to do that though?
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[2] Also, keep in mind that since they have to buy the game in order to return it, the company either gets a low interest loan for that period of time, or they know the money is theirs, and can offer store credit depending upon jurisdiction.
[1] I can't argue percentages, but possibly close to piracy numbers. If a person can pirate it for the sake of just wanting to play a game without paying, I can't see why they wouldn't just abuse the system either. It's not going to be large scale like 50% of the audience. Your examples I would have to argue are irrelevant due to the nature of software being a non-physical medium. Sure some stores get the product back used/abused, but that is a physical material that they can still salvage further down the chain in a discount section or something of the sort. It's not a total loss.
Software on the other hand is license only, refund only, no resale or down the line discount or such. The problem would be relieved by physical mediums, but at the moment there is no way to ensure Digital only cannot be abused by such laws if refunds had legal backing.
[2] The problem is more in relation to the developer/publisher and the store front. Sure Steam can offer store credit for the refunding, it's still going to eat up revenue somewhere which the Storefront must compensate in losses. Combined with a legal lawful backing, economic issues that people use to justify piracy and don't care for keeping a particular game... There's nothing to prevent abusing the system theoretically. Low interest loan would be possible, or possibly a "refund tax" which simply compensates for a 'potential' loss in refunds.
Think about D.R.M. Who funds the anti-piracy measures that pirates keep breaking? You, the payer in the price tag.