It doesn't need to be a real problem, for them to make it one.
The immigrant work force will still be there, they'll just be put in camps and forced to work for nothing, while white working class people are sold the idea of "claiming back" "their" land, while the capitalists take it all over in their name (and never share any of the profits or benefits, of course, with a new scapegoat as for why as they need it).
Strictly speaking of productivity without justifying Trump's plan in any way - labor shortage increases productivity. Cheap labor decreases productivity. Expensive labor forces capitalists to invest in new equipment and training to be able to produce the same output with fewer labor hours. Simple example - fastening the same number of bolts using manual screwdrivers in more hands vs electric screwdrivers in fewer hands.
Outside of that, removing a significant portion of the population at any one time would be significantly disruptive in many regards.