My point being that they don't make other cars vanish and they certainly don't make human-driven cars vanish in any sensible timeframe.
On a person-kilometre-minute basis, cars are absurdly inefficient. Stupendously so.
A typical subway train typically runs to about 157 metres. A 2018 Camry runs to about 5 metres. That gives about 31 car lengths for the subway car.
Assume that every Camry is fully loaded with 5 passengers. That's 155 people. In the ideal condition, driving bumper-to-bumper with no other cars, 155 people in 157 metres -- about a person per linear metre.
Here's the fun bit: the passenger capacity of an R142A carriage is 176.
Not the whole train.
One subway car.
One subway car, approximately 16 metres long, holds more people than an entire train's length of self-driving Camrys driving with zero tolerance.
Now add the fact that the trains have dedicated rights of way and note that even with the profound multi-decadal mess made of the MTA you simply cannot replace the subway with any other mode of transportation, not now, not ever. Self-driving cars don't repeal the laws of physics and they don't override Little's Law either.