IMHO the sweet spot is buy new every three years, but when buying get last year's model. Consider two scenarios:

1. Buy the latest model every five years, so your average age is about 2.5 years and your oldest boxes are five years behind new gear. You replace 20% of your boxes per year.

2. Buy last year's model every three years, now your average age is also 2.5 years (since a new box is a year behind the times) but no box is more than four years behind the latest stuff. You replace 33% of your boxes every year, but you pay a lot less per box since you are buying last year's model. And most of the life of each box is near the bottom of the bathtub curve (in my experience, after any box has been in service for 3.5 to 4 years its probability of failure begins to rise dramatically).

It is of course wise to standardize so it's easy to stock spare parts. Also, when older boxes are taken out of service keep a few around for cannibalization purposes until absolutely none of a given model are still in use.