In an ideal and rational world, you are right. However the DACA repeal will frighten a lot of people, and the markets work on emotion. Why cause such a shake-up? Why cause companies to spend money worrying if their human capital will suddenly be rendered worthless?

For good governance, Trump should have proposed a bill to formalize DACA and then rescinded the executive order after it was passed. That would have avoided any monetary losses whatsoever.