I can't say I disagree with Trump here. Do we really want to fight this fight? Do we really want to take deport some teenager whose first language is English and all his schooling are American? Plenty of the Dreamer cases are quite appealing. Do we really want this fight?
Furthermore IMHO a good case can be made that this move will dramatically affect the tone of the conversation in our country from here forward. A lot of the emotion for sanctuary cities is based upon not wanting Dreamers to be deported. Of course the professional left will not change, but the American people will observe and take note.