Note this is very different from: how long does it take for all the devices on the market at the time to become inactive? If these would be equivalent if (1) iPhones were homogeneous and (2) devices had a constant lifetime, but the truth is there are many models available and different devices become inactive at different rates.
Concerning point (1): As an extreme example, imagine Apple releasing a rock solid phone (iPhone RS) with a ten year lifespan followed by a crappy phone (iPhone CR) that dies in a year. Within a year of the CR's release, they'll all be inactive, meanwhile all the RS's will be plugging along (assuming they all last exactly their lifespan). Meanwhile the average lifespan will be identical to what it would be if they were released in the reverse order.
Concerning point (2): These numbers can be skewed by devices with different lifetime distributions. If we assume, realistically in my opinion, that device decay is a Poisson process, different devices with different decay rates mixed together in this analysis would render this analysis way too simplistic.
For consumers who buy the very newest of devices, this isn't a particularly helpful metric because of (1). For consumers who buy older devices, it's not very helpful because you don't know the distributions pointed out in (2).
For financial analysts, however, this is a very interesting analysis because it can be used to make predictions. For instance, if you assume the lifecycle is increasing, then you see a strong incentive for Apple to get their hands on older-but-still-functional devices so they can resell the same phone again and again. My takeaway is I'll keep an eye out for how aggressively they market the iPhone Upgrade Program.