Silly thought, I wonder how hard it is for a fast MCU or FPGA to emulate a SATA III disk interface?
If you could make a mSATA or M.2 SATA module (which are getting rare and expensive) which took a stack of lower-density MMC cards or eMMC chips and put them in a RAID-5 style arrangement… you could re-use existing flash media stock, possibly even replace faulty parts as they wear out.