No one denies the importance of immigration to the economy.  But equally important is a population that respects the laws of the nation.  

Current immigration laws allow for as many immigrants as we desire to keep the population expanding and the economy growing.  They also help assure that the people coming here actually contribute to the economy (with exceptions for refuges) and aren't criminals.

Perception is everything; if Trump continues to rail against illegal aliens, it will have the desired effect of discouraging more illegals from coming, encouraging those here illegally to self-deport and get back in line, and increase public support for enforcement measures to keep would-be illegal aliens out.