While I believe that power supply longevity is better than for hard disks/fans and other mech components, power supplies are still prone to failure and also damage caused by line voltage irregularities. That's part of the reason why serious servers have two redundant power supplies, its not just for battery back-up. Power supplies do fail, that's why they're FRU's (field replaceable units) in datacenter equipment.
Warranties are more about getting people to buy stuff than they are about actual longevity. Very very few consumers will bother to warranty a power supply from a 10 year old gaming rig, for example. Few people excercise warranties period. But they do create a warm-fuzzy feeling for buying decisions and if the margins are high enough for the MFG its not a big risk anyway.
If I can avoid that by not pushing the limits, it's worth doing.
That's why I always invest in a great PSU.
I could suspend that sign overhead with one extremely high quality aerospace serial numbered M2 bolt or six M4 bolts from the hardware store, financially I'm better off with the hardware store bolts.
Do you have any statistics or other sources on that? (honestly curious)
I'd like to see the numbers too.