As an optical media enthusiast, I’ve done a fair amount of research into how, why, and when discs fail. Because the discs use two or more polycarbonate layers pressed together, moisture can sometimes work its way between the layers and speed up degradation, especially if a disc has been overly flexed at the center. Heat and UV can also speed up degradation.

Another problem is that plastic is petroleum-based and it breaks down over time. A lot of people think that the reflective layer (the metal layer) is actually the data layer but it almost never is. The data layer itself is polycarbonate, sandwiched between the reflective layers and more polycarbonate layers.

The newer discs like blu-ray movies are made with better plastics that should last at least 100 years. Depending on the dye layer of writable and rewritable blu-rays, they should last either at least 25 years or 100 years.

What disc is left? blu ray?

When they say plastic takes [huge number] of years to decompose, they're talking about how long it takes to disappear completely. The usable lifetime for most plastic objects seems to be only a few decades. (I don't know about the specific plastic they use for optical discs, though.)

The foil coating usually deteriorates first

Pressed optical discs will last a very long time. The lifetime of burned discs depends on the type of dye that's used to store the data. Many of the early CD-R's would get corrupted after a few years, but that was solved a long time ago.