As long as the platters are intact, a stuck motor will not prevent someone with the right tools from reading data off of the drive. There's an entire industry dedicated to recovering data from hard drives in various states of disrepair.

A bricked NAND chip, on the other hand, is an entirely different kind of problem.

This cries out for a missing product: A modern version of the old disk pack drives. You could pull out a cartridge containing the platters and store it in a hollowed out mountain for a century, but just drop it into a freshly built/rebuilt/exercised mechanism when you need to read it.

6 months is a very short time. I have disks that are without power for a few years and none of them broke after I connected them to a power source.

i'd say that it depends on how you stock them once they're powered off.