Some years ago, I was privileged to engage in a discussion on headphone detection with some Apple engineers, who had clearly worked on the issue for some time, and I learned something surprising:
The 3.5mm headphone jack standard... isn't.
Even after you set aside the issue of cheap manufacturers releasing shoddy products, you're still left with the fact that there is no actual standard dictating dimensions, number of contacts, location of contacts, size of contacts, separation distance between contacts, etc. Different manufacturers can and do make them slightly differently. More crucially, there's also no validation authority to check that your products meet all the specs.
Let's just take the most obvious dimension: 3.5mm. For ages, those phone plugs were advertised not as 3.5mm, but as 1/8 inch (3.175mm). So if you wanted to make something compatible with a "1/8 inch" plug, you might get your dimensions wrong. Apply this principle to every other contact's position and size on the plug, and you can see where this is going.
Moreover, some phone plugs have five contacts (Apple's own, for example). The "meaning" of each contact is not standardized -- that ring in the middle may be microphone input, or the contact switch (answer/hangup) on the cable, depending on who made it and what it was intended to be plugged in to. Further, if the rings in your cheap knock-off aren't lined up with the socket contacts, then bumping the plug could cause the socket contacts to short across the rings, which would get interpreted as a button press, and your call gets dropped.
The result of all this mish-mash was the Apple engineers found designing a (cost-effective) headphone jack that worked reliably with all headphones and headsets one might encounter in the world was simply impossible. You couldn't position the contacts in such a way that they would never short across two rings (some idiot may have placed their rings very badly). You couldn't know ahead of time which contacts did what, and probing at insertion time was fraught with other perils, especially if your contacts created a short across two rings. Despite their extensive research and massive efforts, they still got tons of support calls about how someone's cheap-ass headset didn't work in what has long been assumed to be a standard phone jack.
So my theory is: They declared the problem insoluble, yanked the phone plug, and designed a new digital interface.
An adapter for "3.5mm" stereo headphones will almost certainly be made available. Yes, you still have the compatibility problem with other "3.5mm" devices, but now the problem is in a $30 adapter, and not a $750 phone. It will be interesting to see how liberally Apple licenses their connector so that third parties can also furnish adapters.