It's a good analogy and I certainly accept your point. It could just be a marketing thing though:
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.
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.
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? :)