A couple of problems in your analysis. There haven't been four straight quarters of decline. There have been two! The first ever drop was in April of this year followed by the quarter ending in June.
Second,you are comparing an all time record year with this year. Important to consider more than one year period. For example, the iPhone 7 could have a fantastic Christmas quarter, but still be less than the all time record Christmas quarter of 2015.
The other problem is you that you are making the mistake of the Apple bashers, of which you aren't, in ignoring the history of the iPhone. Beyond its inception, it has always been iterative improvements, of which this year is shaping up to be , if rumors are correct, one of the largest number of improvements ever, to wit: Double base memory, double max memory, increased battery life, stronger, sharper screen with better color, increased water proofing, increased dust proofing, Optical Image stabilization, new low light camera, improved flash, new duo lens system, improved speaker sound and volume, improved microphones, new faster, more powerful processor, new digital lightning earbuds with two way interaction and other benefits, new color choices, and that's just what has been leaked. You may be dismissive of each of these, but most users will find these all welcome improvements and a compelling reason for many of them to upgrade.
Finally, people don't care about the 3.5 mm jack per se, they just want to be able to get good sound or be able to use there "connected devices." A huge number of people just use whatever headphones that Apple puts in the box. Those people will plug in their new lightning earbuds and not think twice about. Another large number never use headphones and they will love the improved sound from the new speaker set up or not notice as they stream through BT. which Apple is likely to announce an improved version of. The other group that has legacy headphones they want to use will put on the adapter and forget about it. All of the above will love the improvements that will come from the lightning phones, e.g., two way interaction, etc., that Apple will explain on Wed, and all of the many other improvements I've listed.
A very tiny number of people, hugely outweighed by those who are attracted to the above, will give up all they love about the iPhone and the benefits of the Apple eco system and all of the new improvements because they are so uniquely wedded to using their legacy equipment without an adapter.
Sales across the industry may decline for all sort of economic and other reasons, but it won't be because of some extreme devotion to a jack.