There are major problems with DEXs from a technical standpoint that make them uncompetitive with centralized exchanges.
Basically, its physics and privacy. Blockchains are both slow and somewhat transparent, neither of which is suitable for an exchange at scale.
So if we wanted a Steem exchange, it would be best to build one privately and go from there.
RE: When Steemit.com becomes the largest crypto-exchange in the world!