If a PSU fails it can take out all of your PC parts.
But it's not like it has a timer inside that goes off after five years, explodes and kills everything.
Most quality PSUs easily last 10 years.
Most quality PSU manufacturers have insurance policies so if their customer has a leaking water cooler, exploding PSU, motherboard catching fire, etc., they can compensate the customer for any damage caused by their components within the warranty period. So if you're REALLY paranoid, don't use a PSU beyond it's warranty period.