Why would declaring it unconstitutional "collapse much of the legal system overnight"?  DAPA, which is DACA applied to a different population, but created the same way, was struck down as unconstitutional and last time I checked, we still have a functional legal system.  Hyperbole much?

BTW, Obama said it wasn't constitutional for him to do it.  Congress has 6 months to create a fix for the dilemma the Dreamers were place in by both their parents and then Obama who signed an order he said was unconstitutional.