HDD/Flash and technological evolution
"Will NAND flash replace disk?" isn't really a valid question - the question is "(when) will NAND flash replace disk for application X?", and there are hundreds of applications out there with different answers.
If disks weren't increasing their capacity/price ratio as fast as flash, the answer would be simpler - given enough time, flash would be better for everything. But they're increasing at about the same speed, so the most storage-hungry applications are going to use disk far into the future, while others tip over to flash sooner. In particular, for apps with bounded requirements, once flash becomes cost-effective then it's all over for disk, because you only get the cost/GB that disk offers if you need an entire disk.
For iPods the answer was 2005, which curiously happens to be the year that 2GB of flash became as cheap as a micro disk drive. Every year after, instead of increasing the capacity of their base model as disks grew, they could keep it the same and pay less for flash.
For a wider market the transition is less abrupt - e.g. what's been happening with laptops in the last 9 years since Apple introduced their 2nd-gen Macbook Air. (about 45% of laptops are expected to ship with SSDs this year; the rest are evidently sold to 4chan trolls who need the extra storage for all the porn they download...)
For just storing lots and lots of data (like that Microsoft OneDrive account you haven't touched in a couple of years, or the Dropbox folder from a project you finished last year) hard drives are going to remain king for years and years, possibly becoming weird and specialized in the process because they don't have to remain plug-compatible with your old machine.
Finally, flash has had the speed advantage since it was introduced; if you have a small amount of data and you need it to be fast, you're stupid to put it on a disk. However as the size of that data grows it starts becoming cost-effective to play all sorts of caching and tiering games, so that you can use disk for capacity and flash for performance.
Biting the hand that feeds IT
