Should I worry about the sun's collapse ?
Double the latencies when on 4 TB capacity ? that's still in the microseconds range...
Mechanical disks need to rely on caches (try disabling the disk's cache on device properties in windows, and see), and it's completely impossible to go beyond milliseconds (head's weight - movement speed - strength are related to materials we have, and discovering such a material (that could make possible to go beyond ms) would change our whole lives (but not for having better HDD, we would have better cars, planes...).
SDD based on nand flash, memristors or whatever else are here to stay. I agree in some scenarios HDDs still perform best... but the trend will be towards SSDs: because (to make SSDs) you don't need to master another completely different technology (to make HDDs), because it's hard to fit an HDD into a SoC...