I don't necessarily agree with this James, you're right in that more solid shells can sometimes end up being sacrificed to save other in poorer condition but that isn't always the case. As cars get older they all start to rust in model typical areas ( FB's in the radius arm mounts, floor pans, sills, etc etc..... the first areas we'd check when buying a 1st gen) but there are always certain cars that buck the trend and rust differently and although still beyond economical repair can still be a rich source of rare and difficult to source panels. Remember that silver series 3 I had a few years back, the one with the terminal roof, A post, screen frame and bulkhead rot? That car had only been used on the road from new for 8 years and then stood for the next 20 years unused on someones drive. As it hadn't been dragged through salt laden winter roads for most of it's life the floors, sills and usual rot areas were excellent but as the drain channel had been blocked during storage the corrosion happened in other areas. The car was still a scrapper but the shell still had plenty of useful panels and sections on it that went on to help keep other 1st gens on the road.
The life a car's led, where it's spent its life and how it's been stored ca, as we know, massively affect how rusty it ends up.
Panels and repair sections are notoriously difficult to fabricate, even for professional restorers with all the gear, which is part of the reason bodywork costs so much to restore if paying some one else to do it. For the home restorers like us by far the easier, cheapest and best way to get a repair panel is to cut one out of a donor shell. These panels are made on the original pressing so are much easier to fit and if unpicked along the factory seams then plug welded into place the results can excellent. I did that with Spitfires for years.
Unless a car reaches an extraordinary level of value ( like an E-type.... when did you last see one of those being broken for spares?) there will always be some cars being broken and the parts to keep others on the road and in one way that's a good thing. As manufacturers stop supporting older models that have been out of production for years, unless the model enjoys thriving aftermarket support, 2nd hand parts / panels / repair sections cut from donors will be the main source of bits to keep our cars on the road. We're almost at that point with FB's.