There may be a snag to that....

The amount of rope between the glider and the car is shortening all the
time, and the pilot is alive at the start of the tow ?

That looks similar to the winch situation to me: in both cases the
effective rope gets shorter during the launch.

That is quite true, except with the winch you have the full runway length
of rope between winch and glider.
With the Car, you can only start with half that distance from car to
glider.