I’d like to bring another aspect of the story to your attention: Imagine a person, who needs a new phone and kind of is into ecological buying, but then that person learns that the Fairphone runs old software that was released in late 2014. Google stopped releasing security updates for Jelly Bean after 2,5 years. They could do the same for Lollipop, which would mean that in May 2017 there would be no security updates any more.
Fairphone is completely at the mercy of Google here and they always have to fear that older Android versions are cut off by Google without prior notice. There is no Long Term Support version like with other popular software products (e.g. Ubuntu).
A second aspect I’d like to mention is the perspective of the smartphone industry in general. Fairphone started out to change the system and has made some impact and earned respect from mobile operators. What will the industry say, if Fairphone can’t keep up with the fast pace? Nice little project, but they can’t even update their OS to a newer version.
Since Fairphone themselves ruled out newly engineered phones every other year, they have to provide “new” phones by other means, for example with software updates.