Unfortunately making the battery removable will make the phone considerably thicker and probably easier to break which is not what most of the users want
The Galaxy S5 was the last of the mainline series to have a removable battery, and was thinner than the S9 which came out four years later. It also had a pretty good water resistance rating.
Any “downsides” to a replaceable battery are a myth.
Well also the internals and the battery itself is more loose so it’s less resistant to high G-forces
I’m assuming you must be referring to modern phones here right? I never needed a case on my phone until we started getting into S8 territory when phones became incredibly flimsy and fragile. My Note 4 was plastic and aluminum and survived tons of drops. Same with the LG V20 I replaced it with. New phones are required to have a bulky case added on, which defeats the purpose of making them thin and using glass construction. Also modern phones are way thicker than the older phones with replaceable batteries even without a case on them.
Iirc, removeable batteries make phones harder to break. If you drop them, the back cover and battery come off, reducing the shock on the display.
Breaking a modern screen by dropping the phone on a flat surface is not that easy. What can break more often is the electrical circuit between the battery and the phone (causing it to force shutdown that can lead to potential software instabilities), the back cover and sometimes the display flat cable thing