Doubt this is 100% the case. Developers and Apple will still be aware of the earlier phones for longer than you think. Devs especially, since they want to sell the most widgets to the most customers. With the iphone 5-6/6+ having 1GB of RAM, you can bet it'll stick around a bit longer. No doubt, though, that it will be shorter. I'm betting with the iPhone 7, we'll see what you're predicting take place.