Do you have any examples in mind of 6-10M cities with stress-free comfortable commuting and affordable housing for all? Off the top of my head, but without a lot of first- or second-hand experience, Rome seems like the commute is probably a lot better than the larger cities, but my perception is that the affordability is pretty poor? Otherwise most of my experience is in larger or much smaller places.
Something that I think often makes this discussion tough is that there are a LOT of well-known historical European cities that are at under-2M population that I don't think Americans typically realize are THAT much smaller than, say, an Atlanta. I think the challenges of serving a growing city of 5M+ are much harder than a well-established old city of 2M.
Internationally the statistics depend quite a bit on how the data is collected and aggregated, so do it's hard to international comparisons accurately.
I will say using US definitions, commute times are very sticky around the 30 minute time period. Longer commutes and people have a large incentive to move closer, short ones and they don't generally bother.
So in the US Tulusa Oklahoma population 400,000 (1M metro) has a 20 minute commute and NYC population 8,800,000 (20M metro) is 50% worse at 32 minutes average and ~100 cities between those extremes. https://www.titlemax.com/discovery-center/planes-trains-and-...
Edit: This suggests allowing people to move easily move around the metro area would meaningfully lower the need for transportation infrastructure. I suspect NYC has issues with people living in rent controlled apartments having long commutes but being unwilling to leave their cheap apartment, but don't have data backing it up.
If you want stress free comfortable commuting, you're going to need to build an efficient way to move people.
Cars are not really an option when it comes to moving people en mass. It's just too low capacity.
Rome's metro system in particular is stymied by buried Roman artifacts and laws for archaeology. That's not really the worst thing, given that NYC subway construction cost are some of the highest in the world.
That said, I saw what Atlanta looks like. Aside from down Atlanta, a lot of Atlanta is literally just low rise, even downright suburban sometime. It's a smaller city than people thought, given that only half a million people lives within its border proper, but nonetheless traffic is somehow a nightmare.
The big question is what counts as a city. American urban / metro areas around a central city are often large, because there are no other major cities nearby. A similar area in Europe may contain several independent cities. For example, if you take Atlanta with wide enough borders to get the population to 5+ million and drop it in the Netherlands, it will probably cover Amsterdam, Rotterdam, The Hague, and Utrecht.
Something that I think often makes this discussion tough is that there are a LOT of well-known historical European cities that are at under-2M population that I don't think Americans typically realize are THAT much smaller than, say, an Atlanta. I think the challenges of serving a growing city of 5M+ are much harder than a well-established old city of 2M.