When CD-ROMs were new, most people's hard drives were a fraction of what could be held on a CD. The first computer my family had with a CD drive had a 250 meg hard drive. When you could start burning CDs for realistic prices, the average hard drive was probably a few gigabytes; you could back up all your data on two or three CDs.
When DVD burners became available, hard drives were usually a few dozen GB; it took somewhere around 10 DVDs to back up all of your data.
When Blu-ray burners became available, it wasn't uncommon for hard drives to be 500 GB, so 20 Blu-rays to back up your data.
Yes, Blu-ray burners will become cheaper, and yes, blu-ray discs will become cheaper, but by the time they do, we'll be seeing 2 and 3TB hard drives for $100. The $/GB of Blu-ray might drop below hard drives for a while.
Then, hard drives will continue to advance with Moore's law, and by the time the next generation of optical discs come out (which will probably be 150 GB/layer, based on the ~5x ratio of each disc type to the previous), you'll be able to buy 2-digit terabyte hard drives for $100.
Conclusion: Blu-ray is already obsolete, at least for data archival. Hard drives are going to win for the next few years.