unless you live in a densely-populated areathis argument does not consider that if there are no cars, we would need a lot more busses and trains, and then even in a less populated area, public transport makes sense.
only extremely sparsely populated areas would need individual transport.
planned with public transport in mind
why does it have to be planned ahead of time? you take any area, count the traffic, and replace every 20 cars with one bus. you'll find that in many areas that's enough to need a bus every few minutes.
then you take every busy highway and replace it with trains. again, you'll need multiple trains per hour to accommodate everyone.
even in a country with very good public transport like germany, public transport is only 10% of all personal traffic. if all cars were removed, there would have to be 10 times as many trains and busses to make up for that.
a local train that now runs every 20 minutes would then have to run every 2 minutes. instead of an hourly long distance train, you'd need that same train to run every 5 minutes. a small village that only has one bus per day, would still be able to now have a bus per hour. probably in that village many would still keep their car, but they would use it less.
more public transport means a higher frequency, and the whole problem that makes public transport unattractive goes away.
the problem is not the end scenario. the problem is how to get there from where we are now.