I suppose I don't plan on things to fail, but I do have replacements on hand because I know they will. I run an Iwaki external pump as a return; it may outlast me. However, I know that is the single item failure (I don't run dual returns) that could kill my tank so I have a replacement on hand. Everything else is redundant or just stuff that is "nice to have".
I have two extra powerheads I purchased when Tunze stuff was on sale. I run two heaters, each with their own controllers backed up by an Apex, and one extra heater on hand. I have a couple of the cheaper probes for the Apex (pH, temp), also purchased on the rare Neptune sale. I'll replace VHO bulbs on schedule, but the Kessil A360 LEDs (I have 3), ATO pump will get replaced as they fail. My family has a 7 Kw Generator in my garage that I can run to my house (which always reminds me to have it wired with a disconnect to my main CB box). Stuff that is nice, but not essential, is easy to just put on a use to fail program; i.e. skimmer pumps, reactor pumps, ATO pump, heater, lights, bulbs etc. For the most part, I can live without all these things until I can purchase a replacement. We all used to use timers for lights, we used to test pH and Alk regularly every other day, and had what we thought was enough flow. Do I really need to run a reactor or skimmer or powerhead 24/7? I
I also keep an emergency kit on hand for my wife consisting of a roll of Flex Seal and an index card with the number of my LFS in case of a leak. I travel frequently and have alarms letting me know when a pump, heater, fan etc. falls off. All that can be replaced over days. The return pump is the weak link, but with a solid external, I think I might just replace it at 10 years. Lights, reactor and skimmer pumps, life, can all get a mention. Murphy is still lurking, but I feel like I can travel, and sleep well.