wiki
"Another limitation is that flash memory has a finite number of erase-write cycles. Most commercially available flash products are guaranteed to withstand around 100,000 write-erase-cycles, before the wear begins to deteriorate the integrity of the storage."
On average if the write-erase-cycle is 100,000 I'm sure that would go fast compared to a platter drive. The Flash also writes slower. Mechanical hard drives will stay till they think of something more cost effective or flash gets cheaper and more capable.