For me, I don't think anything can replace the physical sensation of putting a key in a lock and turning it. That is, the interface feels very "real" and solid. There's something very satisfying about that.
Likewise, as I've gotten into the habit of giving the handle a strong pull after locking to make sure it's locked, I don't think a proximity system would work so well for me.
My 3 year old Mazda uses a key fob. As long as the key is on my person, the doors will unlock and the car will turn on without me needing to press any buttons on the keys themselves nor do I have to turn my key in the steering column. I have had to use rentals from time to time and the entire act of pulling my key from my picked is so inconvenient and annoying (I know, I know, FWP.)
Likewise, as I've gotten into the habit of giving the handle a strong pull after locking to make sure it's locked, I don't think a proximity system would work so well for me.