The SEC will not be able to go after every DEX, too many will be completely out of their control. They cannot charge someone who is on the other side of the world. DEX's can also become more and more decentralized until there are no central entities in charge of running the exchange or no one knows who these people are.
It depends.
Going after those behind these DEX's is only the first step. Step two might very well be that authorities go after Vitalik & Co (obviously assuming they run on top of Ethereum). It doesn't really help that Vitalik admitted this year that he would write a hardfork if he was forced to do so.
I strongly believe that we haven't even remotely seen the true nature of authorities when it comes to enforcing their age old laws, which is exactly why I am glad that none of this garbage is running on top of Bitcoin. The less you start messing around with authorities, the less you have to worry about.
The only viable DEX is that with second layers we all have the ability to be an exchange ourselves, and that with just our wallet clients. We'll be there eventually.