Bisq is a decentralized exchange, as you brought up, and the developers have done a great job at maintaining security, privacy, and decentralization in all facets of the application.
Bisq's "main purpose" as I see it is to ensure decentralized fiat <> crypto trades. They have systems of arbitrators to make sure that fiat trades go through, and they also have set fees for trades.
As for decentralized exchanges, there exist many from Bitshares to Bisq, but they haven't caught on as most of them are dodging the real problem. Rather than arbitrating with the blockchain, they use different methods; Bisq uses a decentralized team of arbitrators, while Bitshares uses derivatives (or mock cryptocurrencies) that replicate the real blockchain cryptocurrencies. Bisq is not at all a bad application though, and I do recommend you get the hang of it if you're interested in that.
I think the "true" Bitcoin DEX will come out soon once the Lightning Network has established itself. After the Lightning Network release, an HTLC-based Bitcoin DEX would likely work well, and I think development will begin on that very soon.
As for Bancor, BNTs are extremely tied with the Ethereum blockchain. I believe Bancor will only work primarily with Ethereum ERC-20 tokens, which gives them the "boost" of using built-in smart contracts. Bitcoin doesn't have native smart contracts, which is why a Bitcoin DEX has become a harder puzzle to solve.
I think the gist is that there have been decentralized exchanges for specific financial sectors. Developers have targeted places where it's easy to establish a DEX; but a blockchain-backed Bitcoin DEX is the next step in ensuring decentralized trading.
A big thanks to you, mooncryption, for an authoritative response to my questions and what I accept as great guidance about my up-coming time investment in preparing for the demise of the exchanges. I am already Following you, and will now try to do a decent up-vote. If my SP is too weak, I will do so in response to a future post by you.
I certainly recommend to the other crypto enthusiasts in Steemit to keep track of your work.