Audio is so simple to transmit in high fidelity that it can be done with a wire hanger. Using a powered, high-bandwidth data interface adds nothing but cost and complexity, and the space you save is minimal
Data doesnt need to go to the headphones, nor do the headphones need to send data back. Digital-to-analog conversion has to be done eventually, because speakers dont produce sound waves with 1s and 0s. Whether you do it a millimeter from the speaker or three feet away in the phone itself is not important. It wont affect the sound quality.
The 3.5mm jack is robust, familiar, secure, well-documented, and so on weve seen the argument play out over the last year. You know why its good: because it works reliably, worldwide, and with millions of devices. Without Apples permission.
See, its that last part that must bother them. The idea that someone, somewhere, is doing something with an iPhone that they havent anticipated, like making a thermometer or payment system or 3D scanner. Someone who hasnt paid for a license to attach that thing to their phone.
Apple is taking the first step to make sure that never happens. Theyre able to do this because no one can do anything about it. Theyre in a position of immense power and theyre using that power to eliminate something good and replace it with something that makes them money. Its not a conspiracy theory, its a business plan.