I don't see this happening because they made such an emphasis on the fact iOS 9 is a much smaller download than iOS 8 also the fact that they announced app thinning as a thing, which can reduce the size of apps considerably because they only download the assets for that particular device... Makes a huge difference as (for example) iPhone 5 users won't end up having all of the retina iPad and 6+ assets on the phone which are considerably bigger!
Between these things I think the 16gb is almost certainly going to be kept for another year at least!