A well engineered power supply should last in the neighborhood of 15-20 years with reasonable function, as long as it was not run continuously at its limits and/or heat-soaked for a good portion of its life. I'm continually baffled by the notion that they should be any less reliable. I have 40 year old computers working fine on their original power supplies... and 60 year old amplifiers and other gear. This is not to say that every PSU will last for ever, or that even a good quality one won't have some outliers, but in general there's a lot of know-how out there around doing AC/DC power conversion properly.