That is a very long way in the future IF it can ever happen. Decentralized exchanges cannot compete with the volume of centralized exchanges. The reason comes from the incredibly slow transaction times of currency of today. Transaction times need to be milliseconds for decentralized exchanges to be a reality. There is no such currency in existence. Even transaction times of a few seconds are orders of magnitude too long for realistic trading to be viable. Bitcoin, for instance with transaction times of minutes to hours just won't be usable in a decentralized exchange for trading. Another reason decentralized exchanges cannot compete is that centralized exchanges drive transaction costs to zero as the internal transactions withing the exchange don't carry the burden of the currency transaction fees. That alone will kill decentralized exchanges for trading purposes.