Quite simply it is a matter of how much revenue an operator can get in, versus how much it costs them to operate (fuel, maintenance, depreciation, training, etc). This applies all across the aviation spectrum.
So if, after a few years, you can find a more efficient way of doing things, do so. But if the most efficient way turns out to be keeping on the old fleet, that is equally possible. Delta (ex-Northwest) DC-9s have become the classic in recent times, with a 30-40 year old fleet which, when I rode in one last year, Albany to Detroit, was clean and tidy, and gave no sign of its age at all. But I have been in a 4-year old 747-400 whose interior had not been touched since it was built, and was tatty all round.
Those DC-9s keep on going but there are MD-90s, a recently built descendent, a fraction of the age, which have now been scrapped as nobody wants them any more.