Surely an array of redundant flash chips?
Surely someone could make a "HDD" that simply takes SD cards in an array, & handles the wear leveling at a higher level (as well as the in-built on the individual cards)
You plug in a few ?Gb, ??Gb or ???Gb (micro?)SD Cards, and the controller, in the form of a regular HDD sized thing, gets on with it. Uses JBOD architecture or some fancy RAID, according to your tastes, & presents as a standard SSD/HDD.
Completely removes a single point of failure too as if the controller dies just put the SD cards in another controller. If any one card dies, you get an alert and you swap that card.
This already exists, doesn't it? (It's too simple and obvious)