Delta don't SAVE anything by having less GUC in rotation! They restrict inventory for GUC to only routes where the seat is expected to go empty. So people with GUC move to those flights, or GUC don't get used.

GUC is NOT the limiting factor here. It's inventory, controlled by Delta.

I don't know anything about Skybonus so not able to answer your question, but you still haven't provided any good reason why DL should do this.