This is going to be a trend with Apple. They have a good thing going with the iPhone and iPad devices which are both non-upgradeable: consumers are compelled to buy a new device every couple of years or so. It's pretty clear they're getting rid of the Mac Pro, which is highly configurable (with memory, storage and graphics cards) and thus has a much longer life span. So is Apple aiming for a future where you have to…
This is no different than any other phone, it has nothing to do with the upgradability (which arguably is better for Apple than other phones since Android's OS upgrade path is slow at best and absent on a lot of devices).
Phones get upgraded every couple of years because the technology is changing so quickly that there are large differences between the models and with carrier subsidies the upgrade cost is minimal. If the iPhone had a removable battery or a SD slot it wouldn't cause people not to upgrade to the next model.