Do you mean 2.5 or 3.5? Given you say 'back' I assume you mean 3.5mm. I ask as I always wondered why phones didnt move to 2.5 as they got thinner. This used to be the old nokia standard 10+ years back and I wondered if it would return.

That said I feel the better solution for iPhone (and more likely) is to put a lightning cable port on each end of the phone. This way people can use the port on top/bottom as suits their sound/power/data needs and it would suit a button-less screen only phones symmetry. Also it would let Apply designers satisfy the clear market demand while the're egos are maintained (which while unnecessary is often a factor in these things) by not saying they made a mistake and putting the jack back, but finding an alternate solution. A better one too I suspect.