Yeah, "DVD rot" is a thing, but so far it doesn't seem like a widespread problem at this time.
Most cases are likely due to a small manufacturing defect where the disc isn't properly sealed and air creeps in and oxidizes the aluminum substrate that holds the data.
Some discs, particularly DVD-14 and DVD-18 discs, two sided DVDs with one or two double layers, seem more susceptible to rot than DVD-5, DVD-9, or DVD-10 discs, but it can strike any DVD.
Also note that some entire runs of certain DVD titles have this problem. A decade or so ago, a number of Anchor Bay discs were afflicted with DVD rot due to using sub-par materials in the manufacturing process and certain titles were almost guaranteed to become unplayable. I remember Kentucky Fried Movie and Heathers being mentioned as problematic discs.
I still have DVDs that I bought fifteen years ago that still play fine and I've only encountered one DVD that actually became unplayable: The MGM SE of The Howling, which was a DVD-14. I've also had a couple of adult DVDs go bad, but some of the adult companies use unauthorized replicators to cut costs so it shouldn't be surprising that manufacturing defects would creep in.
On the plus side, it appears that if a DVD is going to 'rot,' then it will probably start exhibiting symptoms within a couple of years. That's been my experience, anyway.
The jury is still out on how long we can expect DVDs (and blu-rays for that matter) to last. It's entirely possible that a 'typical' disc might only have a twenty or thirty year lifespan; they could also last for a century and outlive us.
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