The idea of what 'lasts' varies with vehicle use and region.
In this case, it isn't rust coming through from beneath. It was water from above if I understand correctly.
Add that to exposure to dirt/mud (or lack thereof) and proper storage, (not parked outdoors or on gravel/dirt) it's not the same issue as, say, a guy from Maine or Iowa who parks on a gravel driveway, drives down dirt roads, and keeps a car outside under tarp or carport.
Body stuff is darned funny like that, really I can't explain it. I've got a truck that used to need yearly inspections in taxachusetts... more on that one because it reminds me of a story or three. Back in the 90s they made the new rule about rust... any visible perforation or sharp edge was an automatic failure, regardless of area. That reminds me of 3 funny stories that led to on our cars..
1) a 1986 f150. It had rusted out the bottom of the tailgate. The sticker monkey walked around the truck, noted 'rust perforation' and put a rejected sticker on it with the retest deadline. So, I backed the truck out of the bay, took the tailgate off the truck, and pulled it back in for the retest. Tailgates aren't required equipment. Because there was no tailgate, and now no rust, it 'passed' the retest 5 minutes later. And then back on went the tailgate.
2) a 87 suburban. It had a small quarter sized hole behind a rear wheel, through the metal edge only - not into a body void or anything. Same story, it's perforation, the metal could be sharp, blah blah blah. So, left, went to Walmart, spent $20 on a piece of screen and pail of bondo. Slapped it on there not even grinding or cleaning anything. Drove back with the filler still warm from the hardening going on, but, firm to the touch. Not snded or primed, just raw. Not only did it pass, but the same patch is exactly the same way 20+ years later with no worsening of the rust. Surprising for a truck that spent every day until 2019 parked outdoors since then.
3) an 87 cutlass Supreme. They didn't like the chrome splitting on the rear bumper with rust stains and some flaky rust poking through. This time stopped by the local PD and grabbed a free 'dare' bumper sticker. Slapped it over the rust. Now there wasn't anything 'sharp' to the surface, and no hole because the backside was covered by the reinforcement. They tried looking, much to their disappointment. Another 'pass'.
So of the more redneck fixes to rust that really didn't fix any of the problems... the the surprising part about story 3 is how the repair that wasn't a repair has been extremely durable. I guess of it works for where you live, and what you expect of the vehicle, go for it.
