The cold data thesis hasn't worked out as well in the market as many storage vendors would have liked.
When the density of data on disks gets denser, it does so on both axes - x and y, or radial and tangential. But read/write heads only access one track at a time, so the speed-up is only along one axis.
Think about CDs, DVDs, and BDs. The discs can only be spun at a finite rate (somewhere around 10k RPM) before they shatter. The R/W head can read the data within any single track at full speed, but only one track at a time. From CDs to BDs, the amount of data per track increased (this doesn't affect the amount of time needed to read all the data from a disc), but also the number of tracks increased (this does affect the total time).