To your last point, it definitely doesn’t seem to be about making the iPhone thinner — as many have pointed out, the iPod Touch is thinner than the iPhone and still has the headphone jack. The depth issue brought up by John Gruber and others seems more interesting — if/when Apple makes an iPhone that is edge-to-edge screen, internal space will be even more at a premium.
Why have two ports (Lightning + headphone) when you can have one? Because of legacy? That doesn’t seem like a good enough reason and again, would seemingly be holding back potential innovation. (Equivocation needed as these are still just rumored products/changes, of course.)
I understand that change is hard. It always is and always will be. But I also understand that people always move on and adapt faster than they think they will.