Take a look at the bottom of your iPhone (if you've got one). The headphone port is significantly taller than the lightning port. At some point, even if it's not this upcoming generation, the devices will be too thin to support a headphone jack.
I imagine the next line of MacBooks shipping with lightning ports onboard. Possibly even to replace USB (along with adapters) - USB is the tallest port on the recent units.
Removing USB would be a very, very crazy (and bad) move.
http://www.engadget.com/2008/09/23/confirmed-t-mobile-g1-has...
But Apple is not most phone vendors.
Similar thing happened with iPod peripherals when Apple brought out the 30-pin connector about 10 years back.
Then they can sell two pairs of headphones to every sucker!