I was given a Canon AE-1 which was my uncle's. My aunt was having a clear-out and she felt I would make best use of it. It had been stuck in a cupboard for twenty-nine years, untouched. The 6V lithium battery (I was expecting the worst) had a potential of 2.2V. Bought a new one, popped it in and 'Voila!' Looks good so far. Five shots of a test-film used. Really the seals need replacing as they're a bit gooey.
Anyway... It's plastic, dressed up to look like metal. My CD player, bought around the same time, my Casio FX-81 calculator and my Hi-Fi amplifier, again, of similar vintage, have ALL exhibited plastic degradation where vital components have taken on the properties of crumbly cheese. The plastic of the AE-1, as I understand, is ABS. If PVC plastic comes into contact with polystyrene, say, it will degrade faster and quite ferociously. Not sure of ABS properties. But it doesn't bode well for longevity of plastic components, of which I've been suspicious since I was eleven years old when the new locomotive for my train set failed after a few weeks: the motor drive worm, made from plastic, wore out rendering the loco useless.
So plastic cameras will in fact wear out. They'll go brittle, crumble and eventually disintegrate. And it won't take as long as you think.