I would agree, a properly stored belt won't deteriorate in quality for a very, very long time.I just cant agree with any decomposition in a pristine belt! Entropy perhaps? Thats another topic.
It was not exposed to U.V Radiation, only perhaps to temperature changes.
After all it still smells new and is flexible.
Belts deteriorate over time due to temperature changes, exposure to other chemicals (oil leaks is a classic one), and static positioning under tension (car not being driven), among other reasons. If you keep your unused belt in a constant room temperature, away from sunlight or exposure to chemicals or other environmental contamination, it'll still be good to use for a long time. Yes, rubber and plastic do deteriorate over time as the volatiles escape but that is a slow process, particularly when storage is proper.
That said, even if the belt is properly stored I'd get a new one if it's more than say seven or eight years old.