For example, Blu-ray discs are about $0.03 per GB, where a USB flash drive is approximately $1.00 per GB. Using flash memory for WORM style storage (like backups) is just silly.
Not saying _nobody_ backs up to Blu-Ray, but I'm guessing they are in the 1% of the population these days.
What I'd really like, is for someone to come up with a practical form of Archival Media. Something that you could put on your shelf, and _know_ it would still be readable, 10,20,30,40 years from now.
I often wonder whether these guys are for real: http://millenniata.com/
On the other hand, you need 30 DVDs/month (assuming daily backups, and a backup takes ~1 dvd), time to write them, and a space to store them. Also, it is way less reliable.
Also, you wouldn't actually do a full backup every day. You would perform data journaling with periodic snapshots. For example, with daily backups with a change rate of 5%, and monthly snapshots, you need only 2.5x your data size per month ignoring the minimal transaction overhead.
If we have the optical technology to render a certain number of floating gate transistors in a certain area, we also have the optical technology to render 10-100x as many bits of raw data in that same area.
The "blu[e]" in blu-ray is a reference to the shorter wavelength of read lasers (relative to CDs/DVDs) necessary to cram more data on a disc. Trying to cram 100x as much data onto a single optical disc layer would require use of a laser with 1/10th the wavelength that blu-ray uses today, which would incur all sorts of additional costs and complexities. Granted, it might be possible to use other techniques (deep layering, holographic storage, etc.) to side-step such problems, but who knows what the limits are? It's telling that there isn't much research into next gen. optical storage compared to other storage forms.
Additionally, increasingly people are embracing form-factors for computers that do not accommodate optical drives (smartphones, tablets, ultrabooks), which helps drive R&D money into the forms of storage that are more popular.
My point is that one cannot firmly say, a priori, that one format will "never" be cheaper, larger, more popular, etc. than another. Technological progress is rapid in storage.
Because optical storage as a technology (R&D) has died, with optical drives starting to be removed from laptops and soon-ish desktop PCs in favor of flash, and mainly internet downloads/uploads. Bluerays are a dead end -- movie distribution will be (is) on the cloud. Ditto for music. Ditto for data.
Even HDs are on the way out, to be replaced by SSDs in the coming 5-7 years.