My understanding is that the headphone jack actually occupies a lot of space. If you have opened up a phone before, it is evident that all internal space is _very_ well accounted for, so removing a headphone jack is actually a very tangible increase in usable internal space. And it's not just the jack, you need to include a DAC in the device, too!
This means being able to include more/new co-processors, different/better microphone technology, etc, not just thinner.
Only what I've heard from other hardware engineers though, I do not work directly in the space. "Sell headphones," could absolutely be a goal.
The earpiece and speaker distort the sound so much it doesn't really matter what DAC you drive them through. ARM microcontrollers tend to have built-in DACs that are probably good enough for that kind of use.
But when you have your high-quality headphones on, you can certainly tell the difference between a $0.10 DAC and a $5 DAC, just as you could tell the difference between 8-bit and 16-bit samples or 22kHz and 44kHz sample rate.
To get good sound quality, you need good DAC and audio amplifier chips and well-balanced circuitry around them. That takes space.
But then there's that one guy on youtube that managed to not only fit a headphone jack into an existing iphone but also a bunch of circuitry needed to switch the audio path.
If a bloke with a little knowhow and a dremel can do a good job at it so can apple.
If nothing else it proves that the jack removal was not done for space-saving or anything other than business reasons.
However whilst I'm prepared to think ill of Apple under pretty much any circumstance - I'm utterly clueless why so many other vendors jumped in (as they don't have Beats to schill).
Still convinced I'm missing something
clutches onto OG Pixel
So make the phone 0.5mm thicker or so, and you can make the battery thicker and shorter to make room for the headphone jack (with higher capacity than it had before)
I don't know who is pushing for wafer thick phones that are increasingly fragile with shorter battery life.
This means being able to include more/new co-processors, different/better microphone technology, etc, not just thinner.
Only what I've heard from other hardware engineers though, I do not work directly in the space. "Sell headphones," could absolutely be a goal.