I'm pretty sure my comment will be extremely unpopular. But my personal theory about storage capacity is that there are four categories of entry-model users. One group is comprised of those who don't need more than 16GB, because they really don't use their iPhone for very much outside of mobile internet access and probably don't enjoy consumer tech in general. The second is comprised of people who are new to smartphones and may upgrade in the future when they understand their actual needs.
The last two groups are comprised of those who simply won't spend more money no matter what. Enterprise, for example, will just opt for the cheapest option by default. Regardless of what the users/employees think about it, the corporations or small businesses are the actual customers in this scenario. If they could get an even cheaper model without, let's say, a camera or activity sensor, they would choose that one instead.
And then there are individual users who can't or won't spend the extra $100 on an upgrade, no matter what they're getting. If they get 16GB, they'll fill it up, constantly delete unused apps and old photos, and complain about the storage capacity. But, if they got 32GB instead, they would end up doing exactly the same thing — fill up available storage capacity, delete content, and complain about the storage capacity.
I think it's an important point to consider, because the tendency to consistently spend as little as possible does affect entire industries, and it can downgrade the user experience for a lot of people. For instance, the reason airlines increasingly suck is that most people will always just pay as little as they can. They will complain about the lack of food service, about the shrinking leg room, about getting nickeled and dimed with baggage fees — but none of that actually changes their spending habits at all. When it comes time to plan their travel, they're still looking for the cheapest flight that will get them where they need to be when they need to get there. They want amenities, but they refuse to pay for them en masse.
As a result, proportionately, most seats are there to accommodate those who spend as little as possible. If even 10% of airline passengers wanted to upgrade, they wouldn't all be able to. Furthermore, even if they end up spending more by choosing the cheapest ticket and paying baggage fees, compared to getting a higher tier ticket with free checked baggage, a lot of those people would choose the former, because psychologically, they aren't thinking ahead about what is really the better value.