Chapotraphouse

Not too confident, but from what I've read, the THAAD system is on the more advanced side of a web of missile defense systems (another defense option being Patriot missiles). Looks like it offers better defense against short- to intermediate-range ballistic missiles specifically. I think other, less advanced interceptors can still challenge those types of missiles, but probably at a lower success rate. So the effect may be making it easier for Iranian missiles to get through, but it's not a complete destruction of any U.S. ability to intercept.

It may have second-order effects if the U.S. planned on these systems lasting indefinitely: maybe they stocked up on THAAD missiles they can no longer launch, and have comparatively smaller stockpiles of other interceptors like Patriot missiles. Maybe those less-effective missiles will be spent more quickly if they successfully intercept less often, but I'm really guessing at this point.

Also found this:

A THAAD battery consists of 90 soldiers, six truck-mounted launchers and 48 interceptors – eight per launcher – one TPY-2 radar, as well as a tactical fire control and communication unit.

Really makes me question the total of 6 U.S. deaths reported so far.