I'm baffled that we still are thinking that we want thin phones. We have a gigantic camera bump that would be removed if the phone was thicker. Who wants a razor in their pocket, like really?
Not only that, but if phone batteries were user replaceable, they could be sold in multiple capacities. So people that want "thin" phones can keep their camera protrusions, and the people that want more battery capacity could get a pack that allows the phone to sit flush on a table.
A thicker battery would need a new back built with a hump or standoffs across the whole back to increase the overall thickness to match the new larger battery thickness...
A modern cell phone is very different from the old flip phones and SLRs/large dedicated camera. Resealing modern phones after upgrades/repairs is one of the harder parts of the whole process with fiddly little sealing glue gaskets to maintain IP ratings.
I've had a HTC HD2 that had exactly that[1] - an extended battery with a bump. It looked silly, but did its job and in daily usage wasn't that big of a deal, actually provided a resting spot for my index finger.
Literally win win win.