People would probably complain if they add cars in the way you suggested but don't make them available for use for home lots. I've seen a few people upset because they can't make realistic garages for their sims, and I think if they did implement it as a race track it would feel like a big middle finger to players who just want to make a simple house and garage.
One way they could implement cars is like the Sims 2: the player didn't have control over where the car could go (unlike Sims 3's open world), but you could see your sim enter into a car and drive off down the road when leaving a lot. All they'd need to do this is have a car model (they already have several in the game, driving around streets in the background), then create an animation of a sim entering a car and one of them exiting it. They could even recycle old animations from the sims 2 and 3 if they really had to.
I think if they were going to add a race track, they might as well add this in too, as more players have been asking for cars than for racing. I doubt they will add either though. It's just too late in the Sims 4's life cycle, and it's probably seen as too much effort for something so small.
