Current systems are optimized for current constraints. When a constraint changes, the entire system changes.
You can keep saying "Physics! Physics!", but the entire point is that once you change the one input in the physics equations, you can change all the others to keep the balance.
A change in one constraint might allow you to move to a different point in the design space, but it doesn't change the physics by which the system must abide.
More concretely, replacing a human driver with an automated one isn't going to change all the physical constraints that make it inefficient to have cars that cruise at 100+ mph: http://energy-ecology.blogspot.com/2010/05/optimal-vehicle-s.... Whether you're in a Honda Civic or a Ford Explorer, doubling speed from 90 kph to 180 kph triples fuel consumption.
Even if self-driving cars can be a little lighter, that's not going to change the shape of the curve.
Current systems are optimized for current constraints. When a constraint changes, the entire system changes.
You can keep saying "Physics! Physics!", but the entire point is that once you change the one input in the physics equations, you can change all the others to keep the balance.