[istjohn]

The economy isn't zero-sum. Additional workers require additional housing, food, durable goods, entertainment, services, etc. creating additional demand for yet more workers to produce those goods and services. The economy isn't an assembly line with X number of positions to be filled. It is dynamic and growth begets growth.

In fact, as birth rates decline in the first world and countries' populations age, immigration is becoming an important lever to maintain a sufficient supply of labor to fund retiree's pensions and healthcare.