The real problem with the idea of a decentralised exchange is not the technology, its the users.
Users do not care about decentralisation at all. The only thing they care about is convenience, fees, speed and security. DEX has great security, but fees will always be higher and speed will always be slower.
Correct, users don't care.
Plus, trade speed will be the critical problem that will prevent any DEX from seeing high volume, especially in the future.
I've spent quite a lot of time looking at ways to achieve high frequency trading (HFT) that is both secure and reliable (trades don't go missing, no "queue jumping"), and I don't think its possible to get < 5 second easily without some form of semi-centralization. When you compare traditional HFT exchanges such as the NYSE and others which are able to process trades sub-millisecond, well, our decentralized counterparts are never going to be able to compete as a serious alternative.
The fastest I've been able to get trades in our DEX with sufficient reliability is about 10 seconds, but that comes with quite a bit of overhead across the board in terms of bandwidth, processing and other costs.