Yeah, I'd love to have something like that for every phone. They aren't completely the same, but they are close enough for me. I don't think I've seen any other phone with a choice like that, though.
You can get extended battery packs for most phones with a removable battery. I had a Galaxy S2 a few years back that would last 4-5 days of moderate use on a charge with one of them, though it made the phone about 1.5cm thicker.
A) I don't want to buy something to upgrade my phone. I'd rather it come like that in the first place.
B) Everyone says "nobody wants a bigger battery", but if everyone just buys them after the fact, how do the manufacturers know? Sounds like they are just passing the buck
C) Many phones don't have a removable battery, or it voids warranties to do so.