But, at what point do you start drilling rivets and removing panels to inspect structural components within the wing for corrosion or fatigue, where the need for inspection plates may not have been anticipated given the operational life expectancy?
Happily, using the Basler converted DC-3 as an example, the wings are structurally entirely either inspected or rebuilt, as well as lots of other structure. None of these planes was designed to last this long, and happily, the authorities have imposed aging aircraft inspections which are very effective. That said, the creation of those inspection programs is costly, and unlikely to be worth the effort for uncommon types (like a B-17). Though I have zero experience with the B-17, I have come to understand that WW2 military aircraft were built to fight, not last. For the work I did on the Bristol Bolingbroke, it was plainly evident that that plane was not built with corrosion resistance in mind, and why would it be? If it lasted the war, that was enough! Perhaps the B-17 was built with longevity in mind, I don't know. But, it is certain that it was not built with longevity into 202 in mind! The maintenance regime to maintain airworthiness ramps up as age increases, and at some point, maintaining the plane to "commercial" standards is no longer cost effective. That, is for the market to decide, but it is not up for quality compromise....