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.
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.