In response to my post on planned obsolescence, some have pointed out that a good is composed of many different inputs. If there are differences in the quality of the different parts of a good, then it might be rational to reduce its lifespan.

That is a important possibility which I should have considered. Imagine that good 1 is composed of parts A, B and C whose lifespan are 1, 2 and 3 years. The manufacturer of good 1 will converge one the lifespan which, in relative terms, will maximize his profits. If it costs more to bring part A to the lifespan of part C than it is to bring part C to the lifespan of part A, then a lower total lifespan would be appreciable.  That decision reduces the lifespan and the marginal cost which means that a greater quantity of goods can be consumed than if…