This is exactly why I think the iPhone SE will be updated with the latest components each year. Right at the time that the flagship phones starts to tail off, the newer SE would begin production which would allow more of those component assembly lines to keep humming along. This would help justify the millions upon millions that Apple's suppliers invested in tooling for the latest chips, camera, etc. I'm sure these guys are coming to Apple saying "You twisted our arm to setup for your peak volumes, now help us keep the plants running year round".
I would be on-board with the idea that Apple was going to update the SE every other year if Apple was actually designing new components specifically for the SE. But they are not. This is a parts-bin phone. The idea that Apple would update every other year only makes sense if no other product line shares the capital costs.
Also, in the past, Apple's seasonal swings were somewhat mitigated by the iPhone's meteoric growth curve. As the world reaches saturation, these swings are going to be come more severe. Apple is trying to be proactive and minimize the damage by staggering the iPhone and iPad release schedules.