Thing is, NAND capacity isn't doubling every year - more like 18 months or so. The Touch is like the last-gen one, in that there are two spots for NAND packages (each of which has 8 NAND die in it), and the biggest NAND packages money can buy are currently still only 16GB... so 32GB is your maximum.

The only way to get 64GB right now is to find space for 4 packages, or a total of 32 NAND die!

Hugo