IIRC the expected lifespan of a brand new drive in cold storage that has been written to once is 10-15 years. The platters were technically fine after that, it was the lubricants and the screws rusting that caused them to fail. The latter of course is heavily dependant on where they are stored.

Also this single bit of "anecdata": I was once told that in long storage attraction between the electron orbitals of head and platter could gradually drive out the air gap, leaving them bonded too tightly for the drive motor to overcome.