I'm not quick to second guess experienced structural designers as to their choice in aluminum thickness. Indeed, ad good resign can be made bad by going one gauge thicker. There are many cases where the gauge of aluminum chosen for a skin, is more based in it's need to be stiff for ground handling loads (people leaning, and bumping things against the fuselage), rather that flight loads.
Consider the venerable, and arguably heavily built Cessna 206. Some of the fuselage skins are only .016" thick. You don't hear people saying that 206's are flimsy....
Similarly, those "well built" Rockwells, have have aileron skins, which are surprisingly thin, but it's what the plane needs to do the job, and be balancable. Just watch out for hangar rash!
The Twin Otter is one of many mid sized aircraft which have life limits imposed. deHavilland in the day, really was not expecting those aircraft to make it through to the limits, but imposed them, and the aircraft was largely intended as a military design. Most common is a 33,000 hour and 66,000 [landing] cycle service limit, though certain conditions could increase or decrease these values per aircraft. I believe these numbers were reached on analysis early in the designing, and then supplemented by testing, once the aircraft were in service. I have personally seen a wing strut strut/spar attaching fitting in which one of the two laminated aluminum plates was seriously cracked, and a definite safety concern. Happily, the wing had been removed from service already. I was aware of deHavilland engineers having informally said in the 1990's that had they to do it again, they would not have made the Twin Otter wing life as great as 33,000 hours. This supported the DH position of not endorsing extending it. There were other STC efforts to extend and relife the wings. "Extend" was a stop gap, which sort of helped, but the relife STC became the much more viable action, for a long term safe fix. In the late 1990's, the high time Twin Otter had more than 109,000 hours in service. This is much more than DH ever expected.
It is the continued use of aircraft, well beyond the manufacture's vision for the aircraft, whic his introduction thee concerns. The buzz word is "aging aircraft". It's not finding it's way down to our little GA types yet, but it will in the long term. Events like the Aloha and Southwest B737 upper fuselage skin failures are really a big motivator for this new policy. In those cases, like the Comet, it's cylced pressurization loads, but flight loads can be every bit as harmful. Piper PA-28's did have a wing attach AD decades ago, but it was eventually recinded, for lack of actually finding a problem, in any other than the one aircraft which started it all.
The ideal, is to assure that the design of the aircraft is such that detail inspection, or obvious external distress will indicate an impending structural failure, before it becomes a safety hazard. There after, the aircraft is a good repairable design if the repair or replacement of these vulnerable parts is easily accomplished.
It is this easy repair, which makes the much maligned Cessna spam cans an excellent choice for low cost longevity. The Cessna structural repair manuals are comprehensive, and simple. The majority of most light Cessna airframes are folded sheet metal (as opposed to extrusions (more often found in Piper low wings), homeycomb (Grummans), or non metallics of some of most recent GA aircraft.
When a maximum time in service for an airframe is expressed as a "limitation", take it seriously. This is akin to time/cycle limitations on dynamic components (mostly found in helicopters). These "limitations" are much more serious, that the "recommended" intervals for overhaul which manufacturer's state for piston engines
It has been said about structures: "Them that bends, don't break". Perhaps over simplified, poor grammar, but there it a worthy theme in there...