I remember reading somewhere about the design philosophy of the first Fords (this is from memory so it may not be entirely accurate): everything in the car should break at approximately the same time. If the life of two parts is vastly different, then either one of them is not good enough OR the other is too good and therefore too expensive.

The other thing I heard about this, is that if one part is vastly better engineered than its neighbour, say it's more rigid, then the neighbouring part will wear out faster than if everything was cheap and flexible -- 'cos the flexible part takes all the wear.

The story I heard involved Ford engineers picking through dead Fords in scrapyards, seeing which components hadn't ever failed, and then deliberately making those parts cheaper and more crap in the next production run.

Peter