"Keeping older devices available for cheaper" does not quite solve the issue, because Apple is also currently selling the iPhone SE. Both the SE and the 6s are down-market, legacy devices. In my view, there shouldn't be more than one down-market, legacy device.
The author could've selected screen sizes as the premise for Apple offering more choices than before, and the article could've been kept mostly unchanged. Mentioning the headphones jack is just fitting the data to what he wants it to mean.