Well you don't need to do it all at once, and you don't really need to bulldoze every building and road. It would be more realistic to just "upzone" as the other commenter said, and stop accommodating car transport so much and start aggressively redeveloping after adding public transit infrastructure (which would be a lot cheaper and easier if you bulldoze along the routes so you can do cut-and-cover instead of digging tunnels).

While you do that in the core, the outside of the city still grows, and that's all car based - there's no way around a city expanding and not building it for cars, unless you get the bus infrastructure out there before the people show up, which would appear to be a monumental waste when every city in the world's focus seems to be cutting taxes.

The cities in America grow because there's various incentives for them to do so, and nothing really constraining that growth in a way that favors cars.

Don't build new roads to the core, maybe even take out, restrict, or slap huge tolls on the existing roads, and you'll keep people from wanting to commute in by car. Also remove rules requiring parking spaces, and even encourage replacing parking lots with buildings or parks or anything else, to make car driving the most miserable experience it can possibly be

There's lots a government can do to make people stop driving cars so much, and stop moving out to the suburbs. There just isn't political will (in America) to do so.