I mean if you can pay an extra $100 for 64gb instead of 16gb, why doesn't Apple have a tiny bit thicker iPhone model that has a much larger battery ala Droid MAXX? Think of an iPhone 6 with a 3000+ mAh battery and flush camera 
And it's not like this wouldn't work for both sides. It would increase Apples profits (just like the extra storage version does) and gives customers that need it the option. Happy customers + extra profits = win.
They wouldn't even need to sell that model right away or stock it in stores, they could simply handle all the regular iPhone 6 orders for the first few months... then offer the iPhone 6 MAXX as an online only purchase so that they don't have to worry about having any wasted stock sitting in stores.
And it's not like this wouldn't work for both sides. It would increase Apples profits (just like the extra storage version does) and gives customers that need it the option. Happy customers + extra profits = win.
They wouldn't even need to sell that model right away or stock it in stores, they could simply handle all the regular iPhone 6 orders for the first few months... then offer the iPhone 6 MAXX as an online only purchase so that they don't have to worry about having any wasted stock sitting in stores.