That is a good way to look at it. Certainly you can spend a stupid amount of money on spares that you are unlikely to ever want.

I would not carry spare alternator or starter motor, but I am not dependent on the engine for electrical power. If I was, I would rather spend the money on breaking that dependency.

In general I guess my approach is to have backups for critical systems, such as self steering. Plus a good set of tools and 'raw materials' including odds and ends of rigging wire, sta-locks, wire clamps, rope (lots of rope), electrical wire, bits of wood, stainless, plastic drainpipe, araldite. True spares are pretty much limited to 'consumables' like oil, filters, belts etc.

Weighing the boat down with a ton of expensive (and fragile) kit seems foolish given that you can never know what you will need.