A GTP is worth saving if you have the ability. It's tough though because there are a lot of super clean southern ones that get scrapped every year because people down south have no respect for a rust free car.
If you love the car and can weld, hell yes, save it. If you just want a clean GTP there are other cars that can be had cost effectively to give you a rust free core.
Bottom line is there is no black and white time to scrap a car, it's a decision based on cost effectiveness -vs- emotional attachment and it's different for every car and every owner.