Yah, everyone assumed that the clear side was the fragile side and protected it, but actually the label side is much more fragile - and unlike the clear side, unfixable if it gets damaged.

But what happened to the promised sapphire (alumina) coating on disks that makes them scratch proof?

Not exactly; the specification called for a plastic layer on both sides of the metal data layer. In cheaply produced media however the top layer (top here = the one with the print on it) was made as thin as possible, or even omitted and just coated with a printing. Also, there were cases where the two layers were joined/glued together inadequately, not properly sealing the metal layer from the environment.

All of this led to the oxidation of the metal layer; rendering it unusable.

Stamped CDs have nothing but some thin protective layers on top of the metal. CD-Rs, at least good ones, have more protection on top and aren't as fragile.