There are almost 5 billion smartphone users; sales of 300 million a year would imply that those are only replaced every 16 years, which is obviously absurd.

On a separate note, speaking of the average lifespan of a phone, I'm fairly sure that with how expensive they're becoming, smartphone lifespans are increasing. Especially with:

* hardware performance largely plateauing (not in the absolute sense, that of "this phone can do most of what I need")

* the EU pushing for easy battery and screen replacement and also for 7 years of OS updates

* the vast majority of phones having cases to protect against physical damage