100TB should only be 5x (i.e. sqrt( 25 )) slower to access each byte sequentially than a 4TB drive (although shingling will screw up this calculation, since it provides more tracks, but not more density on each track).

Which is a long time, but not totally out of bounds for the way we're currently using disks.

The coming decade is going to be very interesting, SSDs are pushing forward on an insane trajectory, which could either fall apart as the benefit of node shrinks disappear, or accelerate through a virtuous cycle as unit volume increases.

It's even plausible that spinning rust is on death's door in that time frame. The price floor of SSDs is much lower than that of hard drives (i.e. how much does an 8gb hard drive cost to manufacture today in volume? As much as a 500 gb drive), meaning that both mildly performance conscious and price sensitive consumers both move entirely over.

Enterprise customers will still chase the price/gb and storage density of traditional hard drives, but without the consumer dollars to invest in R&D, that could start to stall, quickly leaving them with diminishing advantage and obsolete in the blink of an eye.