You are missing the point again. Apple will not replace a battery unless the battery life percentage is below 80%, even if you pay out of pocket. Users are reporting CPU throttling even above 80%. They are slowing the device down via software (hardware is still capable since no moving parts) to preserve battery life without indicating it to the user. Most users would simply replace the battery, but Apple refuses, and suggest the owner needs to replace the device.
So, imagine in your starter analogy, after the warranty, the manufacturer put an undesired effect because your starter was older, then wouldn't replace it at the customers cost...
I was bringing my original apple watch into the apple store 2 weeks after release (because of an AR coating issue), while I was sitting there, I hears a "Genius" tell a customer with a 3 year old macbook that it was getting up there in age, and she needs to back everything up asap as computers of this age tend to fail. He suggested she get ready to replace the computer as soon as she can. Really? A 3 year old $2k laptop is at the end of it's lifespan? I laughed, but didn't say anything.