By your logic, slavery was one of the finest economic policies. Cheap labour, how about free labour? Have we thought of that? Everything would just be free.

In the real world, the evidence is obvious: average productivity/wages drop, incentive to invest in labour-saving technology disappears, and you get multiple decades of stagnation. Every country which had unlimited, unfree labour has had decades of slow growth as a result.

Income growth in the working age population in the US since 1990 has been about the same as Japan, a country which is widely regarded as on the verge of economic collapse. US per capita income is probably 20-30% lower than it would be with first-order effects from immigration, likely much more with second order effects. Under any other circumstances with economic policy elsewhere, the US economy would be growing 7%/year now (and ofc, the answer for Japan's ills is apparently, you guessed, lots of immigration).

China is seeing secular reductions in production costs because of capital investment, not low wages. The peculiarly statist notion of American capitalists that the route to economic supremacy was large numbers of illiterate Guatemalans should go down as not only an economic failure but a moral one (equally of H1B).