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.