Obviously no one's seen it happen first hand. It's a projection based on what's known about the materials and how they're made. Burned CD-R's have definitely been out in the real world for people to learn how short their lifespans can be, though.

Nobody could "prove," for instance, that the Voyager 1 could stay operational in deep space for 47+ years when it was launched in 1977, but the engineers could still predict and they launched it anyway, and it did. I don't think your argument really holds water.