How does it matter what is the lowest storage? What should matter is highest storage because that is the real limitation. iPhone already offers 128 GB which is more than most latest Android phones and definitely more than any cell phone which was available at the time of iPhone 6 was released. No one is forcing anyone to buy 16 GB but the fact that people buy tells that there are lots of people for whom it is not worth extra money for extra storage.
Android and Windows have 32 GB because many of them come with bloats. I have used Android for 3 years and iPhone for 4 years. On Android (Google Nexus) if I leave my gps app open, my battery used to drain in few hours. I had to make sure to kill all unused app all the time to get 1 day of battery life on brand new phone. With iPhone, I get two day battery life running equivalent apps without worrying about shutting down each app every time.
I don't about SD card experience since my Nexus 5 didn't support it but my Windows phone (yes, I have used that too for 18 months), the SD card was flaky. Sometimes it will use it and other time it will just give error that I am out of memory even when the SD card was almost empty.