If Apple moved to 32/64/128, nearly everyone would have gotten the base model. Instead, for the first time ever, people like myself, and even my mother, decided to bump up to the next tier, which is not only plenty of room for most, but 4x the storage instead of 2x (actually, it's an even higher multiplier when you factor in the storage for the OS itself).
In other words, margins.
The actual profits for making people pay so much for 64 gigs is crazy.
For example, in the past I've always purchased the "base" model. First 8GB then 16GB. But now, for me, 16GB is probably 1/2 my ideal storage size, so I'd probably purchase the more expensive 64GB model.
The new "base" model is effectively $100 more expensive.
I have heard the argument that they offer the smaller storage devices for institutional bulk purchase (think high school iPads with a narrower range of use cases and a managed and/or limited base of installed software), but if that's the case they should stop offering those models to consumers in general.
It's not perfect (the camera is garbage in low light), but it has a MicroSD slot, fits in my hand, and costs a lot less. And I can pick what app I want to use for web browsing, mailto: links, maps, etc.
I'm betting Apple finally concedes default apps in iOS 9, but I don't see them ever adding expandable storage.
Remember that hardware sales are what Apple is making the most money off of.