the tldr is that those numbers of 9s are just table stakes. no system should ever lose data due to routine disk failures. so then, as you mention, there's another whole art to mitigating those other sources of problems.
[1] https://www.facebook.com/atscaleevents/videos/17416916227706...
(a bunch of the early MP folks work at Convex now)
As far as I know Magic Pocket has had 100% durability, but that's obviously beside the point.
Should we trust these numbers though? Of course not, because the secret truth is that adherence to theoretical durability estimates is missing the point. They tell you how likely you are to lose data due to routine disk failure, but routine disk failure is easy to model for and protect against. If you lose data due to routine disk failure you’re probably doing something wrong."
https://medium.com/@jamesacowling/how-many-nines-is-my-stora...
I hate the phrase "Our system has over twelve 9s of durability." Amazon was the first motherfucker to claim this, but the other cloud storage folks are also culpable, but at least they mostly had the modesty to add some weasel words like "designed for" and didn't just straight up claim there was less than a 1 in a trillion chance of a durability failure.
You don't have twelve 9s of durability. Your collection of copies of data on the hard drives do, assuming they exist in a vacuum and nothing bad happens to them except the normal sorts of things that cause hard drive failures that are nice and completely independent. But it completely ignores all other sources of problem, and those are so many orders of magnitude more common that you might as well claim "God-given, perfect durability" because it'd be just as accurate.