If battery life is really what people care about, even over features, then everbody would be using dumbphones instead of smartphones. So as much as people cynically complain about companies, battery life isn't what people value after all.
It's not like I get to choose two different versions of the same model of phone, where $a has a bigger battery, and $b has $feature. It's usually a funky mismatch between two different models, with different specs, some of which make it faster, and some of which have a bigger battery.
But I've never seen phones with a 1-to-1 comparison for batteries.
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.
There are plenty of people claiming to want battery life over thickness. And I don't even think a 1 cm thick phone will sell - there's a limit in that.
I'd be happy to buy a 50% thicker iPhone if it could handle a whole day of hard usage.