I don't know what every person intends to do with their car. But, I have a dozen 60s and 70s cars that I will never restore. They get a proper renovation so I can use them. I like driving old cars.
So, if there is a resemblance of the old pan left, trunk or passenger, I grind it reasonably clean, coat top and bottom with rust converter, cover the holes with thin whatever, lay a new tub of fiberglass on it with a smooth resin finish and give it some paint on the inside and under coat on the bottom. It cost about a $100 and 8 hours. You can't tell by looking at it, and it ain't going nowhere in my lifetime.
Then I put the old car into service. I try to drive one of the old a carb cars an hour a day. Just because.