Just a thought to a possible legitimate reason why the jack could be removed.
Apple has been working on wireless charging technology that works through metal (aluminum) stated in some rumours.
Apple doing wireless charging has also been rumoured to hit as soon as the 7s in 2017.
It's currently rumoured for the 7 to move the antennas off the very back of the phone this year.
Having the antenna bands not as spread out (meaning two more solid or thicker bands instead of each too and bottom split into two, basically being 4 bands and spread out more) could have cause interference with the audio coming from the jack.
Having this new wireless charging through metal perhaps could also have caused interference with the audio coming from the jack.
Seeing as Apple doesn't redesign after only one year and they basically use the same chassis for two years. They would have to remove the jack (and move the antennas) in the 7 to accommodate the planned wireless charging planned for the 7s.
TL : DR
Since they use the same design and chassis for two years, the overall design from the beginning has to be done to fit their plans of the second year. So it's possible if wireless charging rumours for 2017 are true and it did in fact create interference. Some changes (nixed 3.5m jack and moved antennas) are seen earlier in the 7 because they are part of the overall design that were mandatory for the 7s.
Some could argue it's a stretch, but it all kinda links a few rumours together IMO. Definitely could be 100% wrong. But it's possible.
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I am sure you know, that it won't be a simple Adapter. It has to be a Dongle with a Digital-Analog-Converter inside. So you will have a DAC inside your phone to drive the Dual Speakers, which is bypassed. Signals pass via the Lightning Port to a second DAC which then have to convert the digital signals into something your ears can make sense off: an analog signal. Surprise: the 3.5 mm Audio Jack provides exactly that.
Sounds not really convenient to me. Especially since we have the option of external DAC already on today's iPhone. So in aboves example with the 500$ headphones: he could connect it via the Lightning Port with his present phone if he desired.
Not exactly directed at you but reading through the threads about the iPhone 7 I have the feeling that many posters knowledge about the removal of the port is:
3.5mm Audio Jack -> Analog -> bad vs. Lightning -> Digital -> good
That the digital signal has to be converted somewhere down the line to an analog one in order to be heard, seems to be 'forgotten'.
I'm aware that a DAC needs to be included in the adapter. Doesn't need to be much larger if at all than the volume/mic controls in the current stock EarPods though. Some headphones already have their own DAC/AMP built into the cables like that or right into the cans (ear cups)