Some years ago a certain car was designed to give uplift by allowing turbulent air to flow under the vehicle like an aircraft's wings. The idea was the car became lighter and fuel consumption would be greatly reduced. Obviously the car could not take off and slowing down the car would settle down.
Cars are designed to press down harder as speed increases, this gives them more stability, making them safer. Letting air under the body to act as lift, lightening the car would be disastrous, as Mercedes found out with their 1999 Le Mans car
https://www.youtube.com/watch?v=wRxRweuiP6s
The amount of fuel required to accelerate the car is dependent on the mass, not the weight and no amount of aerodynamic aids is going to change the mass of the vehicle, also the mass of the car will only really make a difference to the fuel consumption while accelerating once the car has got up to speed far more fuel used is to counteract the effects of wind resistance and friction and the mass of the car will make no difference to this.
Do you really need to ask what happens to any bad or fundamentally flawed designs? they never make it past the drawing board or prototype stages because they are bad designs.
You could say the same about the design of the hovercraft which has no brakes, the steering is completely reckless and no one in their right mind would ever fund it.
It turned out to be a commercial success with cross channel shipping of both human passengers and cargo.
They are also used in swampy areas, and are a weapon used in the military of some nations.
But the hovercraft has a lot of benefits that far outweigh the drawbacks. the idea you mentioned in your OP is fundamentally flawed on so many levels, not least due to it attempting to break the first law of thermodynamics! you can't create the energy needed to create lift for the car without it coming from somewhere and that somewhere will be from increased drag resulting in higher fuel consumption.... not to mention the points already covered (safety, and the fact that decreasing the apparent weight of the car does not decrease the mass so would not result in lower fuel consumption)
You're are correct, "some problems can be overrcome <sic>" but the basic laws of physics tends not to be one of those problems (well not on a car scale anyhow)
not quite sure how you compare a hovercraft with a car, 2 totally different concepts . Hovercrafts operate in large expansive areas not too much to collide with , dont recall ever seeing any on the M25 , cars have to be a wee bit more controlable for obvious reasons.........