Headphone jack represents an opportunity cost - including it means you can't use the volume for e. g. baterry, camera or simply nothing (making the phone smaller).
I don't believe that, either the space is used for something else, or the phone is in some way made smaller.
Fo if you consider a phone with say an 8 hour battery life, I would imagine the extra space of a jack gives maybe 10? 20? minutes of battery life? while adding an extra layer of complexity viz. Bluetooth (wireless interference, potential pairing and charging issues etc).
To extend the analogy, would you be okay if laptop manufacturers removed LAN ports as wifi "works just as well"? (Wifi is likely probably much closer to ethernet, unlike BT/3.5mm.)
BTW, some devices like iPads that don't have cellular slots... have a plastic shim/object in that place. So sometimes it isn't empty space, it's worse - it's (marginally) extra weight.
Additionally they threw in 2 extra side buttons that can be used for the camera but also mapped to other functions—like for me that is the push-to-talk button for Mumble.