Why do power supplies fail?
There is no mention under what conditions (ambient temperature, moisture, load, dust amount, etc) it can last for the given 5 to 8 years. Some equipment are rated in hours total, or years of normal office use when used for 8 hours per day for 5 days a week. Usually power supplies have electrolytic capacitors that just have a limited hours of use, but any component can fail under stress. If you want specifications that define how long a power supply lasts under some given conditions, perhaps search for a power supply that defines them.