Re: there but for the grace of God...
"You clearly don't operate large (as in undred of TB large) data sites."
No, I work on a site with 1000s of TB (PBs)...
"Those figures are highly optimistic at best"
The fact is that overall SATA drives are FAR less reliable than most other drive types. Your personal experience does not reflect numerous studies that back this up.
"Something else will invariably bite you on the arse (MSA1000 array controllers, as a for instance)"
I use more things like EMC VMAX and HP 3PAR that are fault tolerant throughout. We would not use such a crap single point of failure device.
"This has NOTHING whatsoever to do with BER"
It has everything to do with the actual BER. For instance "We find, for example, that after their first scan error, drives are 39 times more likely to fail within 60 days than drives with no such errors." https://static.googleusercontent.com/media/research.google.com/en//archive/disk_failures.pdf
"Once you get into the 6+TB array range there's around a 2-5% chance of _catastrophic_ failure(*) during the rebuild cycle when a drive dies"
Utter dross - completely invented numbers. I have hundreds of disks running in much large RAID 6 sets with zero hard failures ever and plenty of replaced disks over time... the chances of loosing another 2 disks during rebuild are pretty small - I gave an actual calculation above...
". In the 7 years operating these arrays we had 2 total loss events "
These are crappy low end and not fully fault tolerant arrays. Not an enterprise grade solution.
"We also had ~35 drive losses caused by the controller losing contact with the drive"
Quite. Your issues are not due to normal drive failures under RAID 6.