We're talking about 5 years in a row the same entry memory space. While in those years we've gotten better camera's and bigger apps. With means that the files created are a fair amount bigger then they were back in 2010. Android flagship phones all come with 32GB, a chip that would nowadays cost Apple like $1 with probably very little to none extra research and development costs.
And saying that 16GB is enough is just stupid, no it isn't enough and yes from a $700 flagship phone you're allowed to expect more. Upgrading everything but the storage is a dirty way to make an extra $100. Brilliant because they will get away with it, but for us 'consumers' it's just bananas..
I actually don't get it either, and agree with what you've said. My only guess is that the cell companies are demanding such a model to help push a quicker upgrade cycle... and of course that isn't bad for Apple's current spreadsheets. But, it is bad for UX, and that's why I wonder if anyone is home at the head of Apple these days.