There would be a one-generation transition period where a) the Lightning headphone port is unleashed on the world and b) the 3.5 jack remains for existing headphones. Say, the iPhone 7 and 7S. By the time iPhone 8 rolls around 2 years later, they'd get rid of the 3.5 forever.
I doubt it. As long as 3.5mm port remains - there is no reason for anyone to make a transition to Lightning. Besides, the existing Lightning port already supports digital audio output, and there are a few headphone makers that offer it. Only a complete removal of 3.5mm port will force the transition.
I fully expect Apple to drop 3.5mm port in iPhone 7.
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