Optical Disks as Dense near-line storage?

A follow-up to an earlier post [search for 'Optical']:

Could Optical Disks be a viable near-line Datastore?
Use a robot arm to pick and load individual disks from 'tubes' into multiple drives.
Something the size of a single filing cabinet drawer would be both cheap and easily contain a few thousand disks. That's gotta be interesting!

Short answer, no...

A 3.5" drive has a form-factor of:  4 in x 5.75 in x 1in. Cubic capacity: 23 in³
A 'tube' of 100 optical disks: 5.5in x 5.5in x 6.5in Cubic capacity: 160-200 in³ [footprint or packed]

A 'tube' of 100, minus all the supporting infrastructure to select a disk and read it, is 7-9 times the volume of a 3.5in hard disk drive, or each Optical Disk must contain at least 7-9% of a HDD to be competitive.

To replace a 1Tb HDD, optical disks must be at least 7% of 1,000Gb, or 70Gb. Larger than even Blu-ray and 15-20 times larger than Single layer DVD's (4.7Gb).

Current per Gb price of 3.5" HDD's is around $0.05-$0.10/Gb,  squeezing 4.7Gb DVD's on price as well.

2Tb drives are common, 3Tb are becoming available now (2011). Plus it gets worse.

There are estimate is of maximum possible 3.5" HDD size of 20-40Tb.
To be competitive, Optical disks would need to get up around 1Tb in size and cost under $1.

Around 2005, when 20-40Gb drives reigned, there was a time when 4.7Gb DVD's were both the densest and cheapest storage available. Kryders' Law, a doubling of HDD capacity every 1-2 years, has seen the end of that.