Subways won't go away if autonomous cars become commonplace. That would be like saying "well Facebook replaced Myspace so I'm going to reduce my internet speed 75%". You'd be removing a ton of people-moving bandwidth by removing subways. You'll still have a need for surface trains and even busses too. It's easier (and far more efficient) to move people in bulk most of the way and leave them to do the 'last mile' to their location than it is to individually move every single person from their origin to their destination.
If any person in a planning positions thinks that you can stop planning, and supporting, public transportation (especially underground), they need fired.