More precisely, it is possible to replicate the final value of a futures contract (on Bitcoin or any other asset) up front, by trading in the spot market, even

More precisely, it is possible to replicate the final value of a futures contract (on Bitcoin or any other asset) up front, by trading in the spot market, even though a futures contract’s payoff is uncertain until it expires. Whatever the future’s value ends up being at its expiry, you can put together in advance a portfolio of holdings that will have the same value simply by borrowing a certain amount of cash and using it to buy in the spot market a corresponding amount of the asset on which the futures contract is written; if you do this, the value of your holdings in cash (after paying interest on your borrowings) and in the asset are guaranteed exactly to equal the value of the futures contract at expiry, no matter what price the underlying asset ends up trading at then. So if there is any deviation in the value of the futures contract (at any point in its lifetime) from the aggregate value of the cash and assets in the portfolio that replicates its final value, then there is an opportunity for a pure arbitrage. Consequently trading by arbitrageurs bounds futures prices in a tight range determined by the variables that set the cost of putting together this replicating portfolio: the price of the underlying asset, the interest rate on cash and the yield on the asset (if any), the time until the maturity of the futures contract, and cost of transacting in these markets. This arbitrage relationship is about as hard they get, so trading futures is tantamount to trading the underlying asset itself, since changes in price in the futures market will be translated directly via arbitrage into changes in price in the spot market for the underlying asset, and vice versa.