One thing that people often overlook when their vehicle is sitting months at the dealer awaiting repair (due to lack of parts) is that their warranty (base, power train, emissions, or extended service plan) continues to expire with no extension.
You make a valid point - you’re paying for a vehicle that you do not have use of, due to the failure of GM to provide parts and repair in a timely manner. Although many people in the chain do not have any control over it, they are part of the chain designed and staffed by the manufacturer.
I feel there should be a federal law that requires automakers to extend the vehicle warranties by the amount of time the vehicle is spent at a dealer, awaiting repair or parts (other than a reasonable amount of time - usually a few days). They should also have to pay for the loss-of-use of the vehicle when they cannot provide repair parts in a timely manner.