Apple doesn't want people to buy the base model 16GB iPhone. The optimal market target is the next model up with more storage space. Good - Better - Best.

The 16GB model is there to provide a price point and a viable entry level model. My take is that the next model up is the memory size which has been tuned for majority use cases. That would be the 64GB model.

Even if the next model round of iPhone has a 32GB base model, I suggest that the next memory size up will still be the more reasonable choice.