> According to your theory, wouldn't they just sit on their money?
No, because they gain utility from the use of their iPad for the next couple years which they deem to be greater than the dollar value the iPad has lost due to depreciation. Furthermore, if their cash was earning interest they would have less incentive to buy iPads because the utility of 2 years' iPad use would have to outweigh both the depreciated value of the iPad as well as the appreciated value of their currency, which would lead to fewer iPads sold.
No, because they gain utility from the use of their iPad for the next couple years which they deem to be greater than the dollar value the iPad has lost due to depreciation. Furthermore, if their cash was earning interest they would have less incentive to buy iPads because the utility of 2 years' iPad use would have to outweigh both the depreciated value of the iPad as well as the appreciated value of their currency, which would lead to fewer iPads sold.