Mostly because I have a 32g iPad mini 2, and know how much space my documents on that takes.
For a smartphone, the 3GS works fine for email, messages, phone. If I'm upgrading, I might as well upgrade to something that will be able to take a lot of the daily tasks I use my mini for (document storage, web growing, etc.).
Part of upgrading less often is usually that you get a top-end model when you do, so it lasts the test of time.
Either way, I really don't feel like I have to defend my (or my wife's) preferences here. I do appreciate the suggestions that the 6s doesn't suffer battery life detractions over the 6, and that the additional ram really is worth it.