Competition. I bet you Apple wanted to sell that adapter as premium addon, but because of the angry folks talking about it for a while now due to rumors, they decided to include that adapter with each phone.
Yeah, why should we assume that a company that has a history of making unnecessarily proprietary equipment would do so again in the future? that'd be absurd, right?
"Look at all these new great features you get with this update! Oh, btw your old, obsolete headphone jack won't work with this version anymore... but look at all these new great features! It's entirely your decision, but don't you want all these great new features? Oh btw, here's a voucher for a discount on these awesome new air bud 2's!!"
When have they ever done that before? Everything is within the realm of possibility, but just because someone has the technical ability to do something doesn't mean they will.