OK how about this. What spare parts do you carry and why? What spare parts do you not carry and why?
Off the top of my head, filters, impellers, belts, fuses, wire, connectors, electric and duct tape, rescue tape, anchor and rode, propeller (it was on the boat when I bought it), oil, coolant, wooden and plastic plugs in case there's a hole in the hull, bulbs for the navigation lights, repair parts for the head, a bunch of tools and my TowBoatUS card. There are probably more that I've forgotten.
The list of what I don't carry would be everything else on the boat that might break and that would be a very long list. It would also be thousands of dollars and hundreds of pounds. It's impossible to predict just what will fail and when.
In the end, it's balancing cost and weight against the probability of failure and the danger that possible failure might cause. Crossing the Atlantic and cruising the AICW are two very different levels of risk. The spares I carry are based on my evaluation of the risk involved.
I think I mentioned above that on my last cruise, the windlass and potable water system's accumulator tank failed. Who would have had these on their spare parts list? One of the wiper motors wouldn't run and tripped the breaker. I was able to take it apart, free it up and lubricate the mechanism and that solved the problem. Is a spare wiper motor on most spare parts lists?