25 years from now, if half the cars are then capable of autonomous nav-and-park, we could convert some of the excess parking lots to buildings. (Any lot large enough to have “aisles” will also be large enough to build on.)
It's extraordinarily difficult, and expensive, to convert parking garage space to housing - the heights are different, it's not designed with the same egresses, the floor material is concrete.
It also affects what the ground floor of buildings looks like - you can't have ten ground floor pedestrian exits or bike parking in the courtyard if the courtyard is on the third floor.
Just as you wouldn’t use the asphalt parking surface of an open parking lot as the floor of the house, there’s no obligation to use the actual garage structure.
25+ years from now, if it’s more economical to use for housing, knock it down and build high-density housing.
It also affects what the ground floor of buildings looks like - you can't have ten ground floor pedestrian exits or bike parking in the courtyard if the courtyard is on the third floor.