A front spring was broken on the A Class. The guy at GSF asked if I wanted a pair or not, I said 'No, just the one, I'll probably be back soon for the other'. My take on it was that the springs will last such a short period of time they'll not get old enough to differ significantly.
As for cars having a limited life, my view is that providing they last just long enough not to get a reputation for falling apart then that is all they aim for. Look at the incredible service intervals in recent years, to the non mechanically minded or a vehicle manager it's a money saver, if the engine has serious wear in 4 years so what, the warranty will be up, the car sent to the auctions, etc. Quality oil twice a year or more depending on mileage is my choice.
Steve.