My AC is 20 years old, and recently stopped working. After some diagnosis I discovered the run capacitor was shot, and ordered a replacement for $20 online, shipping within 3 (very hot) days.
After some research, it appears this is a fairly common component to go out, and had I realized this I would have just replaced it ahead of time.
This is somewhat SOP with cars. We replace functional, but near EOL brakes, water pumps, batteries, and many other components to avoid breakdown at in-opportune times. I have home computers acting as servers, and keep a spare power supply in stock since this is a component that often fails.
Getting the run capacitor was cheap at $20, but it's often hard to find these specialized components locally, especially outside of M-F 9-5 hours. So the question is, what components of a residential AC might it make sense to pro-actively replace, and when? The run capacitor seems like a no-brainier at $20 shipped, but what about the condenser fan, contact relay, or anything else that's cheap to replace ahead of time, but hard to find when you need it now?
These are VERY easy components to replace if you know what you're doing. Honestly it's far easier than even working on a car. So why not maintain these units ourselves, and leave the tough work (entire replacement) to professionals.