The "best hypothesis"? That's the classic "lock-in" conspiracy theory. And Apple makes pennies on the headphones compared to what they make from the iPhones (and what they could lose in iPhone sales if the removal of the port proved unpopular) for that to be any great motive.
How about: making the phones waterproof with one less difficult to tackle port, less width/height needed for the new port hence thinner phones, envisioning a wireless future (what with AirPods and co).
The fact that there are pushing AirPods (which don't care for the jack at all) and that Apple gives a free conversion to mini-jack with each new iPhone, means that the theory "it's all about selling headphones/licenses for lighting headphones" is moot.