Dr. Evil again?
The flash foundry folk took on 3D NAND because it provided an escape hatch from the NAND scaling trap of ever-decreasing cell sizes eventually to non-functioning flash. But 3D NAND, the layering of many 2D planar NAND chip structures, will run into its own problems. These are to do with wafer production time and yield, and …
It's still denser and easier to do in the factory than have precision mechanical components at these kinds of scales out in the field.
Going to smaller geometry on 3D is a non-starter because not only dies it reduce the number of write cycles and increase errors, but it makes the device significantly _slower_ - that was the other compelling reason to go back to larger geometry.
Don't forget: we _already_ have 2.5" 16TB SSDs and the driving force on cost is availability, not the need to make more layers. Demand vastly outstrips supply and whilst you can add more production capacity, those etching machines don't come cheap.
Not everyone needs flash capable of 100,000 random 4K ops per second, or 3GB/sec of sequential read/write, or with a drive write per day for five years worth of longevity. For a lot of uses 1000 4K ops, 500GB/sec, and a drive write per week or even month is more than adequate, That provides a way to get cheaper flash for cold data, entry level PCs, cheap phones and so forth.
If you can process 5x more terabytes of flash per fab line when making the denser stuff, it will cost a lot less since the equipment investment per TB is 1/5 of what it is on the faster longer life stuff.
Solder the devices on top of one another with only a free select pin to operate the low or high level (literally) device - double the capacity at probably 75% the performance ... with decent caching and a nifty algorithm possibly even better.
If I suggested that was what we used to do in the 80's none of the youngsters would believe me ...
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