I don't think the temperature variation is much of an issue in our temperate climate, Roly; I can't imagine how it would cause the belt to deteriorate. I would lay odds that the cambelt you saw had been broken by an unusual stress, e.g. failed bearing of driven pump/alternator or tension pulley failure.
I have been carrying a spare fanbelt in my TR7 for 30yrs. The original item, a flimsy toothed 1cm wide band, shows no sign of deterioration yet. I suspect my Audi cambelt is made of sterner stuff!! I would be much more concerned at having the original belt replaced badly with an inferior product....there are too many stories of belts failing about 20,000 miles after their conscientious owners had replaced them 'for peace of mind'.
Climate is immaterial when underbonnet temperatures can exceed 100C with a turbo in full throttle.
Modern cambelts are subject to far greater stresses due to often running oil pumps/water pumps and various pulleys. Rapid changes of speed put huge strains on the fabric - acceleration/deceleration.
I have seen piccies of a Lexus 2 litre cambelt which looked fine from the front but the rear half did not exist due to a faulty tensioner... There were no sounds or external signs of problems...
Most cambelt failures after replacement can - in my view - be attributed to non replacement of othet items driven by the cambelt.. Specifically water pumps (VAG) or tensioners which have a finite life almost the same as the cambelt. Fra more likely cause than inferior products.
Malfitting is also an issue.. either non aligned correctly or belt stretched during fitting..
Judging by the standards of many mechanics, I would trust few...
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