By far replaceable batteries. You used to be able to purchase physically larger and higher capacity batteries to get insane battery life, but because they would include a larger rear plastic for the phone it would still look normal. Now we have to waste space and lose efficiency with external power banks.
Pretty sure phone cases with external batteries exist that are literally identical to what you are describing (“purchase physically larger or higher capacity batteries”). Also current phones do a lot more than the old phones you’re describing as “having insane battery life.” Sure, a cell phone of 2005 could be left on for probably two days straight without needing a charge but you were only getting an occasional text message and maybe calling someone once or twice and maybe playing Snake during that timeframe.
External batteries are not the same as there is substantial loss in transmitting the power to the phone, particularly with the many "magsafe" compatible wireless ones. The wired ones add substantially more bulk for similar battery size and although the standard for battery life is much better now, for many otherwise great phones it's still not amazing (aka every pixel prior to this year's).
Being able to quickly swap a battery or simply replace it with a 10000mAh cell for only a few mm more thickness (my preferred method) simply isn't an option now.