Slow (~ 60 rpm), high capacity, 5.25" drive

WD Internal Drives Internal Drives
4

60rpm? Isn’t that basically what tape backup is? (or close to?)

But doesn’t tape backup have quite a bit more density than platters?

Platters are suited to data that is updated regularly and at random… they’re resilient, even when used over and over and over. They have their niche… I doubt it can be expanded to what the OP suggests.