Even if Amazon breach their SLA, I think they only have to refund 10% of one month's bill per year - i.e. a 1% discount. I suspect they'd make a good profit even if they paid out a full 10% refund every month.
Unless an SLA is accelerated - i.e. >100% refund - I don't think it's worth taking particularly seriously.
Of course if an SLA only guarantees 95% uptime, that's probably a big hint to design for failure!
It's like the hard disk maker that gives you a 1 year warranty vs a 5 year warranty... which one believes in their product more? :)
Suppose it's the same hard disk with a black sticker instead of a blue sticker. Drive with 1 yr warranty @ $100, 5 yr warranty @ $150, 20% additional failure rate over the extra 4 years, 50% redemption rate on failed drives. Cost per replaced drives = 20% * 50% * ($100 + $30 processing costs) = $13 = $37 profit.
Totally fictitious numbers to try to prove my point, of course :-) But as the SLA becomes increasingly low in value, the signalling value decreases in my book.
(Edit - fixed my math!)
One of them may be planning to be out of business, sell the HD business unit in 2 years, shove off the risk via financial wizardry, etc.
My guess is the great majority users will not RMA a dead hard drive after 4.5 years regardless of the stated warranty. Even if they did, it would only represent replacement with a future smallest-possible-capacity drive.
While agreeing it's not about the money it's about my site being up, I nevertheless was pretty shocked by this statement.