Widely used by itself doesn't mean it's the right standard for the future, cf. iMac without floppy disk, MacBook without optical drives, iPhones without Flash support - all of which were the most widely used standards at the time.
Agreed in terms of the iPhone, but what about the Watch? The audio jack clearly isn't the right standard to connect earphones to the Watch, which will be forced towards BlueTooth, so forcing the iPhone to go along could may sense
If you look at the layout of the iPhone front screen, the size of the bezel underneath the screen is determined by the home button and the depth of the audio jack. Apparently they can't put the audio jack under the screen because it's too thick. There is a rumor that the iPhone 7 might significantly reduce the top and bottom bezel and integrate the home button with TouchID into the screen. If that's the design they're pursuing, and if it's true the audio jack can't go under the screen, then that's a serious design problem to solve and not just a minimal weight loss.
Bluetooth is the expensive option partially because audio jack is the cheap alternative. If Bluetooth earplugs become the standard, their price will go down significantly. If they go Bluetooth, I would expect that Apple would launch some cheap Bluetooth earplugs (those Beats hardware people aren't sitting around doing nothing I assume).