Actually 16GB is enough for 4000 12MP photos.
At the end of the day it's annoying, yes, but it's the users own fault if they bought a product that doesn't suit their needs.
You can take plenty of pics even with 16GB (but you don't have 16GB of free space, so it is less than 4000).
But think about videos. At 1080p you need about 150MB per minute, so with 10 minutes of video you need 1.5GB
If you are on vacation and take some videos you may have 2-3GB of space just for them, not counting the apps, pics, your music.
Sure 16GB may be enough for some users and if you constantly save your pics and video to the Mac you can deal with it, but is not a great user experience.
When I had 16GB on my iPhone 5 I happened to run out of storage while taking video and I had to frantically delete apps to make room for more pics and video since I was at a wedding party and took a lot of pics and video on the same day. Back then I decided my next iPhone should have at least 32GB of storage, I don't want to spend tons of money on a phone and have to deal with low storage every now and then.