Shock! People on 24 month contract don't upgrade every 8 months
HTC’s One M8 was down 23 per cent on the previous M7. The Samsung Galaxy S5 was 33 per cent down on the Galaxy S4. And Sony launched two flagships eight months apart, but neither matched the popularity of their predecessor; the Z2 was 23 per cent down and the Z3 61 per cent down.
There is a hint up there especially with Sony, but equally valid point for Samsung and others. Most people (if they have gone the more or less subsidised route) are stuck with 18-24 month contracts. spewing out new models constantly doesn't fit in with the contract lengths. Combine that with the fact that mobiles have kind of plateaud a bit with no earth shattering reasons for people to upgrade and that will all hit sales.