Electronics that are running at capacity tend to have more issues. If a 700 watt power supply wastes a couple bucks worth of power a year, but lasts twice as long, who cares about the power?
Power supply longevity is already extremely high compared to any other component, if you're looking mostly at high-quality brands like Seasonic. Their top tier product line (with 650W models and up) has a 12-year warranty, and the next tier down (with 550W models and up) has a 10-year warranty. If you want a long-lasting power supply, you can just buy one directly.
When troubleshooting electronics, in general, power supplies/converters are always the first thing to check.
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.
What's more, a failing power supply can cause all manner of weird symptoms that don't immediately trigger the "bad psu" thought in most people's heads.
If I can avoid that by not pushing the limits, it's worth doing.
So you can over-spec your quality, which is expensive, or over spec your wattage using average quality gear, which is cheap.
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.
A 700W 80+ power supply will generally be more efficient pushing 450W than a 500W PSU because the wattage output is in the mid-range where peak efficiency generally occurs.
