If they're getting what they want and in a welcoming environment, I expect many of them will stay.
Which goes back to the second part of the comment which is that it now makes the federal government an actor or an accomplice in human trafficking.
Not to many think beyond the 'trolling' part to the actual impact. The federal government would be releasing what it describes as 'the worst' on to the streets of this country. It would be taking them to areas that position themselves as 'welcoming' and providing the most benefit. It will shove them off the radar for later apprehension. And it legitimizes the entire 'sanctuary city' action by recognizing the status.
Will Trump be quick to damn his own administration if a MS13 member he freely released into a sanctuary city kills someone, as he does others who have 'let them walk in'?