Find some old phones with removable batteries. They are maybe 2-5mm thicker than the modern phones with non-removable batteries.
Can you find a case that would add 5mm or less to the thickness of the phone and not add to the length or width of the phone?
If the internal battery dies will the case battery still allow the phone to work or are you SOL?
How thin do phones really need to be? Does adding a couple of millimetres to the thickness really mean people won't buy your phone? Of course you won't be able to brag that you have the thinnest phone on the market but again, is that really a bad thing?
There currently really isn't any consumer choice since there aren't any major brands offering phones with an option for either an internal battery or a similar phone with removable battery. If the only "choice" you have is to get a major brand phone with an internal battery or some off brand with a removable battery is that really a choice? Most likely there are MANY more trade-offs than just the battery difference to consider.