I'll make a bet right now that killing the headphone jack will be a step too far and Apple will be forced to bring it back pretty quickly.
I sincerely doubt that. I think the real question is whether Apple sticks with lightning or switches to USB-C. USB-C audio will be standardized. There is a lot of anxiety over the 3.5mm jack now but I feel that is due to uncertainty.
I realize this is a little off-topic, but here goes anyway. My predictions:
1. Phones will transition to digital audio quickly as the product cycle is fast. Most people won't notice.
2. 3.5mm analog jacks will stick around as a secondary option for a while, but eventually disappear.
3. Computers will switch to USB-C too. The 3.5mm jack will stick around longer there because computers tend to be bigger, have more ports, and longer life cycles nowadays.
4. TVs, home stereo equipment, cars, etc. will pick up USB-C digital audio as well (following the now established standard). This transition will be slower than computers, but it will still happen.
5. When this is all over, people will think of USB-C similar to how they think the of the 3.5mm jack now. IOW, a ubiquitous standard that can do anything, why would you want to muck with it. By this time, USB-C will be the de-facto port for video, audio, peripherals, charging, docking stations, etc.
6. If Apple is still using lightning at this point, they are doing it solely to control users.
Now, this is speculative but I do feel that this will happen. It feels a lot like the first round of USB adoption. I remember being upset that firewire didn't take off and fumbling with the various PS/2 to USB adapters, cursing at bad drivers, etc. But USB did eventually replace a group of interfaces and most things work smoothly without hassle. USB-C is not one company's pet, it is being developed and promoted with wide participation from the industry. Lightning is, however, one company's pet and that is the potential monkey wrench in this situation.