The $15 adapter is most likely of similar quality as the DAC and other audio components in the phone, so I don’t see the point. (Also, most people I know with $1000 headphones use an external audiophile-grade DAC anyway.)
This is basically an arrogant move that can't really be technically justified. E.g. By arguing that better external options exist - better external cameras also exist, should we get rid of the camera too?