This would be a horrible move. The iPhone is already thin enough and yes it would be form over function - slimming the phone slightly at the cost of functionality. It truly would.
Yet another bloody adaptor would be plain annoying. Especially a small easily loosable one.
It would be typical arrogant Apple though, moving away from a truely industry standsrd connector to an expensive proprietary one to shave a mm or two off the iPhone's 'thickness'. It would also be a terrible idea for the environment. The mass production of a new adaptor + dumping of old earphones is not exactly environmentally friendly.
I'd like to be able to charge AND listen to music.
I'm already thinking of not buying a new iPhone due to the lack a 4 inch phone, so removing the headphone jack would push me right off the cliff. Listening to music should be easy as plugging in headphones. I'm not interested in stuffing around with recharging batteries, turning bluetooth on and off (the iPhone's battery is already poorly enough without bluetooth on all the time), when a 3.5 mm cable is a simple plug in thing. No adaptors to carry around, no adaptors to loose and nothing to worry about.
Apple can go ahead an introduce wireless headphones or lightning ones