Though, I don't think that large scale, working decentralized exchanges will necessarily come around any time soon since integration with the fiat economy would be difficult. And even when they do become mainstream, "largest economic expansion in history" is far fetched.
Personally, I don't think that the integration of decentralized exchanges with fiat gateways will be difficult
Besides, I don't think it is an issue at all. If you need fiat for your daily expenses (like groceries, at barber's, etc) you can cash out at regular exchanges and exchangers, or use Tether if you are looking for a fiat backup. But for many people, cryptocurrency conversions are the most required operation, and with Bitcoin stabilizing, who would need fiat anyway? Also, if fiat is going to kick the bucket one day (highly unlikely, but still), then this issue will get resolved on its own as you would no longer need it and consequently fiat gateways and exchanges as well