Imagine you're a user that wants a stable value.
You hold some of the bitcoin from your sales (or wage or whatever) and invest some of them in a "1971 dollar vs bitcoin" contract.
The more bitbulls the more you will be able to gain if bitcoin falls. If bitcoin rises, you lose from the contract but gain from the bitcoins you hold, so with the right proportion you stay the same.
Yes, I agree that kind of contract would accomplish what I want. It would not be seemless and easy for Grandma to use, but a sophisticated trader could make it work just fine. Maybe that is really all we need to attract those trillions of dollars, since most of them are controlled by sophisticated traders.
Maybe some later service or client bring it closer to grandma, but that can be outside the chain.
The result of the contract (who gains, who loses and how much) depends on the voting, on the real price of the commodities.
The price you mean may differ is the price specified in the contract as a "draw" where neither party gains or loses.
The voting affects the external price which affects the fees, but nobody forces you to buy or sell at that price within the bitcoin network. If you're willing to pay slightly higher fees, you can trade as far from the external spot price as you want.
But when you send the sell order you don't know which miner is going to solve the next block nor what spot price he's going to report.
Also, voters don't get to vote what they think the price is, they can only vote yes/no on whether to accept the latest block from a miner with the external price embedded. There's not any advantage to voting no if nobody else does. Messing with the exchange rate requires massive collusion with 51% of bitcoin hashing power. That's for sure not the first thing I would do with all that hashing power!
The problem could be the opposite, miners accepting the blocks even when they have wrong spot prices in the fear that everybody will accept it and he's still mining for the old block.