You might be rather surprised, at how slow chemical changes (decomposition) can take place. This may not apply directly to this case, but once upon a time, I had an fishing lure, made of some sort of soft plastic, and I stored it in one of those plastic 35mm film-roll cans. Well, I discovered it hidden in a desk drawer, rather some years later, and was quite surprised. The lure, itself mostly undamaged, had managed to actually "eat" its way though the film cannister, part-way. Almost like something warm melting a block of ice - except this must have occurred over the span of literally years. So the rate of chemical decomposition of some materials can happen over a wide variety of time scales.
I have had CMC-made K-Hypermedia CD-Rs "fade" over a span of about a year before. First time that I had ever seen a long-term failure of a properly-burned piece of media. One more reason to avoid using CMC media for any long-term storage needs. I also have some Ritek 6x-rated Cyanine CD-Rs, that when read in a modern high-speed drive, generate a whole host of C2 errors. Slowing down the read speed to 4x, I can get by with something like an average of 200 C1 errors. But considering the quality of the media in the first place, it's hard to argue that those degraded. If anything, they've held up decently for crap-quality media. The problem is that the top silver layer tends to just flake off on these, taking the data with it. :|
Recordable disc media's dye layer is not 100% stable, and it does degrade over time. Quality discs with good-quality dye, have a decay curve that should allow the media to be readable tens of years from now. Cheap dye will degrade to the point of unreadability in a much shorter timespace. (See prior CMC example.) It's kind of like film. Some companies make really cheap print film, that will fade/brown after only a few years, and better-quality print film will allow it to last for a very long time. But eventually, all film degrades/fades over time.