So, in layman's terms, if I buy two of these and mirror them is it a pretty safe bet they'll last my whole life?
Regarding longevity, often the predicted lifetime of a drive is close to its warranty. You will sometimes experience no issues exceeding design lifetime, and sometimes drives immediately explode. I’ve seen both, from four-year lifetime drives entering year 13 in continuous service to other drives buying the farm one day after lifetime and SMART wear indicator is fired.
As drives age, mechanical disruption becomes a much bigger deal. That rack of 13-year drives is probably one earthquake or heavy walker away from completely dead in every U. Even power loss, including from regular shutdown, will probably permanently end the drive when they’re far beyond lifetime. That’s the danger in a 24x7 server setting if you’re not monitoring SMART wear indicators (even if you are, really); power cycling your rack can, and does, trigger multiple hardware failures. All the time. If all the drives in it were from the same batch, installed at the same time, and an equal amount past lifetime, it’s very possible for the whole rack to fail when cycled — I have actually heard of this happening, once.
MTBF is unexpected failure. Design lifetime is expected failure.
There's always events dear boy, events!
MTBF lets you compare between two drives intended for a similar purpose.
For something that you own at home, the only medium that will last your lifetime in loosely controlled environmental conditions is paper.
Archival CDs might also last a lifetime (regular CDs certainly won't).
But I agree with your point that the MBTF of a single unit is not a way to predict it's lifetime.
It dawned on me when some older relatives died a few years ago that their papers, some very old, survive with a modicum of careful handling. My grandfather's immigration papers, great-grandmothers portrait on her wedding day, etc.
Today, many of us are one expired credit card away from losing all of that in digital form.