Nobody designed them the way they are, at least not with a grand design in mind. Traffic is shaped by planning for existing demand

That is not how it works, at all. They model future demand and they do make executive decisions to shape traffic in the way they want it to be, not just the way it is today.

as long as you don’t have a credible idea why millions of people should give up their homes to live in overpriced shoe boxes without a bit of green and quiet in the city, this will get you nowhere

That is happening because:

• The rest of us are subsidizing their lifestyle through our taxes. North American suburbs don't pay enough to cover their own infrastructure.
• They do not experience the externalities of their lifestyle. It is us living in denser areas that suffer from the increased motor vehicle traffic that suburbanites produce.
• Ever increasing car traffic has led to widening roads and culling of trees. Eliminate car lanes and plant trees, I say.
• Cities aren't loud, cars are loud. Reduce car traffic and our streets won't be noisy.

People love living in spaceous houses they own.

They don't love it so much when they have to pay for the cost of the infrastructure needed to support them. Stop subsidizing suburbs and suddenly people will be much more accepting of more modest accommodations, like most of us do.

Remember that those urban centers would and could simply not exist without people from the outskirts working and shopping in those urban centers.

Plainly false, as those suburbanites could simply move closer to where they work, if only zoning laws permitted them to do so, which is not the case in most of North America.

Again, and it is a point that no amount of mental yoga can get around: what we want is something that already happens in plenty of towns around Europe and Japan that existed before the advent of the car. It is not unrealistic, it is the historical norm.