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 think that's happened in practice. If anything in recent times the smallest phones are getting bigger, and truly small phones (like the iPhone minis) no longer exist.
Possibly not for many phones, but don't forget, the innards of phones get rearranged a lot between generations at times, and the only thing that can easily change in volume is the battery.
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.
Sony’s Xperia line is some of the smallest & lightest flagship phones while having a headphone jack and a microSD slot (& can unlock unlike Zenfones) so I don’t think these are good excuses.
The microSD doubles as a second SIM slot so no, not really. 3.5 mm jack doesn’t get in the way of anything & I use it more than 50% of my time when the screen is on so it more than makes up for the space. I don’t know many devices competing @ 157 × 68 × 8.2 mm (6.18 × 2.68 × 0.32 in) & 168 g (5.93 oz).
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.