Progress
Seeing it first-hand as both the boss and consumer of the work produced, the biggest dent it puts in is career progression for entry/junior US IT grads. Right now they get massively out-competed by more senior folks on H1Bs at the same price point. This is bad news all round mostly as it does limit salary acceleration which would act as the natural corrective and reduces investment in employee development. Couple it with enabling powerful offshore firms and their manipulation of the process and applicants for H1Bs and you pretty much have an entrenched failure that is remarkably tricky to get out of without significant disruption until the skills gap is bridged.