I certainly hope we DO see DEXs take off in the near future, but right now, every single one suffers from a severe lack of liquidity, which distorts prices and can make trades difficult verging on impossible if there's nobody with a suitable counter-trade offer.

Liquidity is the number one reason traders love the Forex markets, and the reason so many Forex traders will trade only in the major currency pairs. Hell, I've even known a couple of traders who will only trade GBPUSD or EURUSD for that precise reason: If the market is sufficiently liquid, there is pretty-much ALWAYS someone who wants to trade at your price.

On the flipside, where liquidity is too thin there's no market, no matter how many bells and whistles the "exchange" has, and no matter how attractive the notion of pure p2p exchange might be.