Said it before and will say it again....16gb is plenty for a lot if not the majority of users who wont post on forums like this. They get a new phone on upgrade via their mobile contract every 12/24 months and the 16gb is a good entry point for these folk who just want an iphone. They will use it to message, odd phone call, email, some selfies, some social media and browing the net...they dont need a load of space to have massive apps, 4k videos, loads of music. Plenty of folk have used the 8gb versions of iPhones without many issues in the past as well.
Also as mentioned the 64gb sits at the same price point i believe as the old 32gb so folk are getting double the space for the same price. No need to ditch the 16gb phone as its used by lots of folk all over the place with no issues.