Were those arrays doing regular scrubs, so that they experience rebuild-equivalent load every month or two and it's not a sudden shock to them?
If your odds of disk failure in a rebuild are "only" 10x normal failure rate, and it takes a week, 5 disks will all survive that week 98% of the time. That's plenty for a NAS.
If the drives are the same age and large parts of the drive haven't been read from for a long time until the rebuild you might find it already failed. Anecdotally around 12 years ago the chances of a second disk failing during a raid 5 rebuild (in our setup) was probably more like 10-20%
My goal isn't to be rude, but when you skip over a critical part of what I'm saying it causes a communication issue. Are you correcting my numbers, or intentionally giving numbers for a completely different scenario, or something in between? Is it none of those and you weren't taking my comment seriously enough to read 50 words? The way you replied made it hard to tell.
So I made a simple comment to point out the conflict, a little bit rude but not intended to escalate the level of rudeness, and easier for both of us than writing out a whole big thing.
If your odds of disk failure in a rebuild are "only" 10x normal failure rate, and it takes a week, 5 disks will all survive that week 98% of the time. That's plenty for a NAS.