You know, I feel bad for making two posts in a row, but I have sorta seen maybe a few clues ...
It looks like the degradation sort of starts in the first 15% in a band which slowly "grows outwards". Sure, I don't have enough bad scans to prove this, but then again, wherever a disc is initially readable, the bad band stays towards the inside, and not the outside. And it might be growing until it engulfs the initial track, thus taking the disc along with it. The surprising characteristic is that the drive is slow and takes a lot of time, but it CAN distinguish the media info block, so it can report the manufacturer AND the BURNT SIZE of the disc. It just cannot read the actual data. To be honest, unless those pieces of data are more protected than the rest of my data, I cannot see a reason why the burnt size and status of the disc can be determined and my data can't. Maybe a media problem, or maybe (in my mind, a more likely case), a strategy problem where the laser power curve doesn't match the disc's characteristics ... and is worst at the particular diameter .... must keep an eye on the still-good discs to determine their failure pattern.
Aside from that, I also notice that bluray burns are sort of ringey as the laser power keeps changing to the conditions of the disc, however, I cannot discern a pattern. All the failed discs - some have 2 different or three different bands of burn darkness at different diameters, but all have failed. Most of the bands are fairly well controlled though - you can really only easily see them under strong LED light ... some of the bands on my TDK discs are worse.
But as you can appreciate, I've lost about 120Gb of archives there, it's very disappointing especially at the price I paid for these discs. I have, to be honest, lost faith in optical storage altogether as most of my CDs and DVDs are slowly rotting away ... as are these BD's ... and I've lost faith in hard drives, having caught "silent" corruption of data volumes over time (there are a few papers on this online which suggest silent data corruption is more widespread than we know of). There is nothing that will last forever, or even last 10 years with a good chance of remaining bit-integral.
P.S. To the eye, these discs are flat, unwarped. They follow my table surface just fine. Most are unscratched, all scratches, if any, are on the outer rim. Visually inspected, the discs are no different from the alive ones, including the code in the BCA. There was no overspeed possible, and no selection of speed at the time - 2x was the ONLY choice. I'd hate to think what my 4x TDK's oversped to 6x might have done. I will find out.