The signed statement is a political argument. A letter from 100 law professors is just a legal argument for one side, not the end all be all or we wouldn't need a Court system. An injunction for DAPA was upheld by Supreme Court with a 4-4 tie, meaning a 5-4 decision against the legality of DACA wouldn't be that far fetched. DACA's legality certainly isn't a sure thing as the law professors seem to indicate.

There is also no question that Trump has the authority to reverse said executive action. I think most would agree (polls spell this out) there should be a legal path for folks that were brought here as children, I just disagree that the current deferred action program is the best or even legal method. The DACA program should be made law through Congressional action signed by the Executive leading to full citizenship. Congress failing to act does not mean the President has the authority to ignore the law and just do what they want. Separation of powers and maintaining checks and balances is too important.