The AX750 wasn't great, I had an AX860i that died and killed a bunch of parts.
I only use seasonic PSUs now.
Yes some materials can degrade even if the PSU is not being used, but unless it's a crappy PSU, it will take decades for the plastics to become brittle or for solder joints to fail.
The first thing that will fail in a PSU 99% of the time is the fan, which is the only moving component.
I think you're overestimating how delicate electronic components are.