I wouldn't be so sure that they will be cheaper to rent than it is to own a car.
Of course they won't be. Somebody has to own that car (ie, the car rental company) and they're going to want to make a profit on their investment. Per mile, it will always cost more to rent than to own.
Now, an individual might -- by restricting the amount that they drive -- not pay as much out of pocket for that car, because they're sharing the miles with other renters. And for some people that might make sense. But generally not for folks who don't live in extensive densely populated conurbations (aka, "targets", for those who remember the Cold War).