Should I Replace My 15-Year-Old Cooler Master GX 750W PSU?

I’d replace it rather than wait for failure. It may continue working, but it’s far beyond its expected warranty and service life, and 80+ Bronze does not indicate that it was a premium model. A decent modern 700–850W PSU is usually inexpensive compared with replacing a GPU, motherboard, or losing data.

It probably isn’t an immediate emergency if the computer is stable, but age alone is a good reason to plan a replacement soon. Capacitors and other components degrade over time, and older units were not necessarily designed around the rapid power-state changes of current CPUs and graphics cards.

You could check the rails with a PSU tester, but a basic test won’t prove that an aging unit is reliable under modern transient loads. The fact that it still works is encouraging, not a guarantee. I’d use it only temporarily and replace it with a reputable current model.

The stuck rear switch is another sign that the unit is simply old, even if it doesn’t affect normal operation. Power supplies don’t have a dependable universal expiration date, but 10–15 years is well beyond the point where replacement is sensible, especially with new hardware attached.

There’s no way to guarantee that a failed PSU will protect the rest of the system. Many power supplies fail harmlessly, but some failures can damage multiple components. I’ve seen a failed unit take out a graphics card, hard drive, and RAM, so saving the cost of a new PSU isn’t worth risking an expensive new build.