The problem is content is taking more space these days. Photos, Music, Video, so they shouldnt offer the 16gb anymore. The Social apps like FB, TW, IG can balloon up to 800mb even...so 16GB fills up quite quickly without people noticing. Games can take 1-2GB even.
It would be like if they had macs with 40GB still since "thats all most people use". 16GB for a phone isn't quite low these days, just look at the iOS8 upgrade problem.
Thats the main reason apple should of had 32 as base instead of 16.