Effectively there is about a 30 to 40 year period that most of these cars would have seen use, though starting in the late-70s some services that they were used for would have started to decline. Newer paint wouldn't have necessarily been a concern as long as structurally the cars were sound and not prone to leaks. And being plug door cars that would have been a bit of an advantage for certain shipments.
I do remember, though, the rapid decline in similar insulated Fruit Growers Express boxcars in the late-70s which happened about as fast as the disappearance of the 40' boxcars occurred which really surprised me but I took it for granted that it was just on my nearby line and not overall occurrence as part of the industry at large. Not being a high trafficked line I can only assume (yah, I know the dangers of that) that it was not a good example of railroads overall.
