Something's theoretical useful life and it's sensible life are different things.
Take an automobile for instance: the only part on a car that can not be replaced with reasonable ease is its body structure- therefore a car that remains rust free and has a very solid structure like a Mercedes can last as long as you are willing to replace parts. But does it make sense to, for instance, over a three year period, spend $15k to replace the engine, transmission and rear axle on a forty year old Mercedes when you can replace it with a safer, faster, more comfortable, more luxurious and much more efficient (say twice the fuel economy) model for $40k?
Secondly, you seem to be assuming that the only reason they are replacing them is wear. Perhaps it offers superior capacity, amenities, efficiency, comfort, or some such like that? Especially consider that superior uptime results in capacity improvements.