Good point. It's kind of like airframes, their life is measured in cycles not hours. Each pressurization causes an expansion and later a contraction. That 737 in Hawaii that blew the roof out yet still landed in the 80s is an example of an airframe being past it's useful life, but they weren't measured by cycles then. Doing island hops it continuously ran short flights for a jet that size and each cycle weakened the airframe until it wore out. It wasn't incredibly old, but it was discovered the hard way that repeated pressurization had a bigger impact than time in the air.I heard that they only did the initial stress test where it applied that level of pressure to the vessel for an extended period of time and it did pass. However, as you mentioned, they did not do any testing for repeated exposures. Since carbon fiber had never been used to build this kind of vessel, there was no real world indicator of how many repeated exposures it could handle. Well, now we know.