Multiple-bits-per-cell is astonishingly involved; with three bits the electronics has to distinguish between eight voltage levels in the face of process variations and environmental conditions that vary widly. There aren't that many electrons in these cells to begin with, and the charges leak over time. I think of the engineering that goes into this stuff as "heroic." 4 bits per cell and you're talking full bull-goose crazy. My hat's off to ya (no, I'm not going to buy any of that).

So you ECC the shit out of things and hope. Or you use less ECC, get more "checkbox capacity" on the box the consumer sees, and most people won't experience failure until they throw the device away. It's all about margins.

SSDs, even the expensive ones you buy, don't have a super long shelf life and are not what you want to use for backup media.