You mean like the Moto Z and the Z force? Z has a 2600mAh battery and is thinner. Z force has a 3500mAh battery for a 1.8mm addition of thickness. Motorola has done this before with the Droid Maxx.
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.