what happens is that the industry goes through certain discrete technical upgrades. for instance, EUV in fabs, or GMR disk heads. none of these are really planned, none of them are exponential. and they usually interact with other phenomena (such as Dennard scaling).
in a sense, the phenomenon is more like "expectations are exponential, and this motivates manufacturers to schedule updates".
hard disks are still improving, arguably similar to how they have in the past, but there are limits to demand. the consumer market has mostly dropped out, for instance due to flash.
even in flash, there is no exponential scaling in devices. people got excited in the initial startup, when for instance, mature TLC is so much better than early SLC. but all that's over: it's both mature and we'll probably never see PLC. even QLC is interesting in that it illustrates that most of our storage is very cold.