Nobody needs to worry about storage density in the sense of how much shelf space you need for cold storage. Tape, NAND flash, and even high-capacity hard drives are all dense enough that the incremental cost of more shelf space is not the most important part of the problem.

And NAND flash is already pretty 3D, with hundreds of layers of memory cells fabricated on the surface of each wafer, and several such dies stacked in each BGA package, and it's not uncommon for U.2 drives to contain two PCBs each with NAND packages on each side.