Whole sections of the country that are zoned for suburban single family housing would not exist as they are today. Not because they’d be illegal or anything, but they’d be incredibly unpopular if most people didn’t own a car, which is needed to basically get to or from a suburban neighborhood.

I understand the question to be something like: what happens if a majority of people are unwilling/unable to own a private automobile. And I think the immediate answer is that suburban neighborhoods cease to exist, at least at the current density levels. Either a neighborhood must densify so that transit options make sense, or they must aim to become rural living. This also means that things like suburban schools either turn into walkable urban schools, or into small one-room rural schools.

I don’t actually think rural living will go away, because the fact is that the grand majority of people – USA and abroad – do not prefer rural living. The 18th, 19th, 20th, and 21st Century trends are that people tend towards urban areas, where services and jobs exist. There will always be people that want to live in the hills on 20 acres, and therefore need an automobile. But that has never been the majority, so if a majority of people refuse owning an automobile, they will also mostly refuse rural and suburban living.

There is no plausible situation where over 50% of people willing decide to: 1) not own a car, and 2) live in a suburb or rural area. This is from the fact that all other modes of transport into a suburb or rural area are either: 1) nonexistent (ie metro rail), or 2) ludicrously expensive (ie Lyft, or transit with 15% fairbox recovery) if the cost was borne by the people living there (as opposed to being subsidized heavily by other taxpayers… Ahem, America).