Instead of witnesses, any group of trusted personalities or companies could form a similar decentralized escrow business that would be better than a lone exchange if not quite as good as an unmanned service...
In fact, if all the businesses that are building on BitShares were to agree to add their reputations to the mix, it might be ideal.
Better yet, if we elected the Top 15 Most Trusted Businesses in the ecosystem to each run a sidechain interface node wouldn't that solve all issues?