The massive expansion in Americans' prisons contributed little to the decrease in crime rates of the past two decades.

US imprisonment does tremendous human damage. Whether it harms the economy depends on who you include in "the economy". It may be good for the plutocrats in general. since some of them profit from prisons or employ prisoners as slave labor, and the rest consider many Americans superfluous. Prisons (which tend to kill prisoners who get sick) may be their method of choice to dispose of the superfluous.