"In reality, the PDs moved to the invasion zone with some delay and disruption certainly, but without serious loss to Allied air."
I understand the argument. I am not historian, but IIRC didn't some at least of the PD's suffer massive armor losses in trying to get into action due to having to travel distances under Allied air attack. Coupled with naval artillery up to 25 miles from the coast, it sounds like no effective defense strategy was possible.