First, it's not single purpose. It's used for right channel, left channel, mic, play, pause, volume up, volume down, answer call, hang up call, next track, previous track, etc. Some accessories use it for an IR blaster. Some podcast makers plug high quality mics into it to record interviews. Credit card swippers use it.
Second, what about it is "legacy" exactly? According
to this article recently, 3.5mm headphones still represent 83% of unit sales last month. That means 83% of people who bought new headphones in June will be disappointed by Apple's move. And probably more from every month before that. That is a large pool of potential buyers that Apple would be alienating.