I'm so looking forward to this and have since the rumors started about this. Here's how I see it, and why I'm in favor -
I'm taking a guess that either at WWDC (which makes more sense) or during the September iPhone event, they're going to announce hi res audio via Apple music. Having used Tidal lossless before (and still would except I like Siri's apple music integration) the difference in audio quality even on 3.5mm is night and day. When using a lightning adapter to bypass Apple's DAC, the quality only improves. Apple being Apple, they won't do lossless streaming (but I'd pay $20 a month instead of $10 if they did), they'll do 'good enough' hi res.
I'm also thinking that the elimination of the headphone jack is future planning and hardware obsolescence. Granted, as of NOW, everyone has a 3.5 headphone jack on their devices and not just cell phones, of course. But you used to be able to play cassettes everywhere too and that's all gone. Apple always gets called foolish each time they eliminate a peripheral connection, and each time they've ended up just being ahead of the curve...which allows them to be on the front lines of establishing a standard that they want to see be the standard. Except thunderbolt. Can't win them all. If you can at least get people thinking about and accepting the fact that they can get audio differently than they used to, just like people were convinced to get data from somewhere other than a floppy disc, or a CD, then you can make people move on to use a new standard. If you lock them into your standard, they'll be less likely to switch later. It's business.
Finally, I think this is leading up to the 2017 iPhone 8. They aren't getting rid of the jack for this phone because, truth be told, they don't have to. It's going to be identical to the 6s save a new processor and maybe another gig of RAM. This is setting up next year's phone. They're going to need that space for more battery for whatever processor they're working on and to drive an (edge to edge maybe?) OLED screen without sacrificing battery life. Plus, I'm guessing for the 10th anniversary phone, they're going to throw in the kitchen sink in order to jump past the competition now that they're moving from a tick tock cycle. They'll need a big boost on year one to keep the momentum through two more years afterwards until the next major redesign.
Just my opinions.
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So by that logic, the only way to make the iPhone thinner is to remove the phone part itself. That means removing the 3.5mm jack is completely irrelevant.
But the 3.5mm is the one component that can't be made thinner. You can thin out memory, shrink a processor, etc. But because a 3.5mm thick piece of hardware has to fit into that jack, the phone can never be any thinner than that. Plus it takes up space in an already cramped phone casing.
