These dual layer rewritable DVDs are DOA technology just as Sony’s DDCD and Sanyo’s HD-BURN
Some factors that caused it never been released:
Manufacture cost is rather high. Dual layer media is difficult to produce. Notice that DVD±R DLs are much more expensive than SL. A spindle of DVD+R DL are twice to third as much as a spindle of BD. The cost of rewritable DVD±RW DLs would be even higher.
Compatibility issues. DVD±RW DL will be incompatible with current hardware. This is due to optical signal levels lower than prerecorded media that will require hardware and firmware to be adjusted to accommodate the differences. Analogous to CD-RW’s introduction in 1996, neither existing DVD recorders nor DVD players will be able to write or read rewritable DLs. History repeatedly teaches that such incompatibilities bring only customer confusion.
Could have been useful if released earlier.
Just like Toshiba’s ED 2.88MB floppy disks: If they arrived earlier, they could have boomed, and still be in use today in secured environments, where today, 1.44MB floopy disks are still being used for compatibility and various security reasons as far as I know.
DVD+RW DL and HVD and HD-DVD still have room however.
Every DVD burner should have sufficient laser power to read DVD+RW DL at least.
HVD: 4TB on one disc for archival? That would be a dream. Probably requires a new UDF version such as 2.70 or 3.00, however.
HD-DVD: Data surface layer in middle.
BluRay has the data surface 0.1mm away from the bottom, which is dumb and fragile to damage.