The recent rise in discussion regarding the limitation of the 3.5mm jack, so far as audio quality is concerned, is more or less pure marketing.
The 3.5mm jack, while not the greatest connector in the world, is more than capable of carrying the current needed by headphones. The only technical limitation there is that due to the common ground, it is not possible to use a bridged output configuration for the headphone amplifier.
Now some informed speculation as to why Apple would like to eliminate the headphone jack:
1) They would like to make money licensing the right to sell Apple compatible headphones and audio adapters.
2) They would like to reduce their manufacturing cost by eliminating the DAC and headphone amp from the phone, passing them on to the consumer who will have to pay for them along with the overpriced and non-interoperable headphones and audio interfaces they will now have to buy.
3) They would like to make deals with car makers and accessory makers to marginalize analog connections as well as non-proprietary digital connections, so as to inconvenience the users of non-Apple products and make Apple products more appealing by comparison.
4) That their partner Intel developed Thunderbolt along with Apple, and that Intel holds intellectual property in this interface technology as well as in USB type C, may have a hand in this from a business perspective.
Thunderbolt is soon to start using a USB type C connector as well.
5) AND YES.. it opens the door for super-high-quality portable external DACS to be used easily, but they can do that now, without eliminating the jack.. for the few people who would actually use them with their portable devices. They would like you to think that this is their primary goal but it is not. If it was, they'd be supporting that via OTG already.
In summary, if/when this change takes place, the difference in quality will be if anything a net loss, since most headphone manufacturers will use cheap DACs in their products. Since headphones break often and DACs usually last the life of the phone, it makes far more sense for the DAC to be located in the phone.