I used to think that was a huge issue. But my friend bought an iPhone 7 and he says it's just not a problem in practice. He keeps the adaptor permanently attached to his (non-Apple) headphones. It's not like you have to fiddle around with the adaptor every time you use your headphones.

Obviously I'd prefer to keep the jack socket on my phone, I'm just saying it's a minor issue. I think people like to focus on it because it feels like the prototypical Apple dick-move, but it's not that big a deal e.g. compared to the walled garden thing.