Let's say that I open up a bitcoin bank, with zero bitcoin, only dollars in reserve, and I sell bitcoin promises. If these promises can only be redeemed for the bitcoin price at the time the redeemer decides in exchange for dollars, then he's just placing a bet that bitcoin will go up; does not affect bitcoin economy in any way.
Actually the bolded is untrue, just as all derivative markets
DO affect the overall market cap and remove the scarcity aspect of the underlying asset.
If there is no derivative then that volume must go through the underlying asset itself.
If the derivative (promise) is redeemable in kind, the emitter (bitcoin bank with zero bitcoin) is going to have a huge problem when the customers cash out (in corn), because the scarcity has been only temporarily suppressed - not removed. Supply crunch and all that. They'll need to buy at whatever the price is. Which means bankruptcy just over the corner. SBF is going to have good company.