So your proposal is, what, you have a warehouse of grain, and each day you sell your grain at price $X and buy new grain (N days more recently harvested) at the

So your proposal is, what, you have a warehouse of grain, and each day you sell your grain at price $X and buy new grain (N days more recently harvested) at the same price? Because you're trading nominally fungible commodities they're priced the same, but when you go back twenty years later you'd have an equivalent quantity of grain to withdrawal regardless of how the price fluctuated in between? The main problem is that you'd need to pay for storage over the 20 years in between, which would take the form of selling some Y% of your grain each year to pay the storage on the remaining percent, which is basically the same as inflation.