Stellar already has a decentralized exchange built in, I've used it a bit and it's really a change to be able to use it directly from wallet, instead of sending coins to an exchange and basically giving them to it, hoping it will allow withdrawal.

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.

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.

I thought so too until I actually used one. The experience is worse than I ever imagined possible (sure there is room for improvement). Additionally the two you mentioned only support ERC20 tokens.