But please note it's not a complete fix either. You still have third parties you must trust (presuming others decentralized exchanges are built like stellar's one). Instead of having one third party for all markets, you have one third party per asset.
When you buy xrp for xlm, for example, you don't actually own xrp, you own credits for the xrp anchor, with the promise you can withdraw them. So it's exactly like current exchanges, except if one anchor gets hacked, it does not affects all assets, just the one it manages. That's still better than current situation, and decentralized markets have many other advantages.
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.
Really? Liquid means trading volume right? Not that others are trading as the price that I find reasonable?
Lets say I find paying $10 for a BTC reasonable, no matter how liquid, I cannot get that deal.
This is the faulty argument in the 'its going to 0' position, there are too many folks like me that will keep buying as long as we can afford it.
There are some dollar-pegged ERC20 tokens like Tether (backed by dollars) and DAI (a stable coin) that could allow what is effectively fiat-value <-> token decentralized exchange, but to acquire actual fiat, that can be used in the rest of the financial system, requires interaction with a centralized exchange that has a banking relationship.