Yes, more people always means more GDP — by definition. That by itself doesn’t mean much for the most vulnerable Americans who are in direct competition with low-skilled foreign labor. The net impact on the country as a whole is often a wash, but the benefits and costs aren’t distributed evenly.
There are other factors, ranging from global trade to automation, that are arguably more important to these American workers than immigration. But there is no obvious way to reverse many of those trends at a manageable cost. Regulating immigration is a legitimate function of government in a way that repealing the field of robotics is not.
The uninterrupted inflows are also sapping many Americans’ sense of national solidarity, slowing the pace of immigrant assimilation and fueling native resentments.