RAID-6 is in no universe a RAID-5 successor. Simplifications of enterprise needs and risk tolerances and compromise acceptance is sophistry. It is telling enough that despite the bluster of some on Hacker News, major storage vendors (ergo - people who know much more than you) still make RAID-5 the default. Maybe they just haven't read the news.
Regarding the backup -- yeah, no kidding. That was the point. If the argument is "this is better because it can accept one more of countless possible failure modes", then "better" can continue indefinitely (why not 10 parity copies?) In the real world of compromise considerations there is a benefit return assessment that draws a line at a probability point.
It also sounds like many on here think you buy a box of disks and then make one universal logical volume on it (e.g. "if you have a spare why not just make it RAID-6?"). Because the spare(s) are usually universal, and you have many logical volumes encompassing RAID-10, 0, 5, 6, whatever the situation calls for.
