The biggest longevity limiter was exposure to UV from sunlight. The second was probably exposure to photochemical oxidant (smog). We had ample amounts of both in Los Angeles.

When I worked at a Peugeot/Nishiki dealership and my wife needed a new bike in 1974, I ordered a bare leftover 1970 UO-8 frame through the shop and installed the original SunTour V-GT rear derailleur and wheelset from my Nishiki, along with a Shimano Titlist front derailleur and Schwinn TwinStik stem shifters, to go with the UO-18 style upright bars she favored. Many years later, after she had switched over to mountain bikes, I recast it as my trusty commuter/beater. I claim to have the best of both worlds, with a European frame and Japanese derailleurs. (The front brake is a Mafac Racer, the rear is a Weinmann Vainqueur 999 clone by DiaCompe.)

I disagree with the UO-8 bashers -- these frames are not bad at all, particularly when compared to mass-produced Japanese frames of the early 1970s.