This debate is usually between "city people" and "McMansion suburbanites", but allow me to offer two alternatives. They are described as utopias, but they really have been realized to varying extents in various places.
1.
One alternative is to revitalize the tons of walkable second-tier cities that are sitting there rotting, and to connect them with intercity transit.
I love NYC, but the problem with concentrating everyone there and in SF, is that it drives up real estate prices and creates a desperate crush to acquire property. Yes, you can say "YIMBY", but even if that were politically feasible, it couldn't keep up with in-migration if there were only those few choices (NY and SF). It would badly bifurcate society, between those who had managed to secure a toehold, and those who hadn't.
(In fact, it creates a culty Bitcoin-like dynamic, where those who have sunk the cost to be inducted (by buying property) are then incentivized to preach the benefits and keep the values up.)
Then, because people would be so desperate for money to live in these places, they'd waste their lives on corrosive professions like surveillance advertising, instead of doing work that actually helps people. That may sound familiar.
And, most importantly, geographic concentration would, arguably, go hand-in-hand with economic concentration -- with the formation of economic "chokepoints" and the concentration of monopoly power.
2.
Even the medium-urbanist decentralization of (1) would however leave people dependent on monopolies for the provision of basic ecological services.
What good is it to concentrate in cities, if all food is produced by one or two huge agricultural conglomerates? This is a condition of total dependence.
As an alternative to urbanism, Permaculture-type people do have interesting ideas. But they require land. Their methods also -- let's be honest -- come with a dramatic reduction in what we call "living standards". But if "degrowth" can come from an increase in autonomy, then that might be a trade worth making. It's one I think young people would consider.
We would probably see some amount of homophily among people, if they reorganized along these lines. There are many incompatible tribes:
- The Colorado ski-bro kombucha-chick tribe.
- Various Black Israelite and Egyption-religion back-to-Africa tribes.
- The Kanye West cult of vaguely Trump-sympathetic heterodox permie stuff.
- Indigenous revival movements. They're likely to have different language but similar interest in the environment.
- Various trad-religious tribes, like the Mennonites and Amish, but you could totally see Benedict Option Catholics or even Evangelicals going this way. Again, they'd just talk a little differently.
They wouldn't combine as a single "diverse" community, but you could see any and all of them borrowing permie ideas from one another.
In the past I have spoken in favor of universalist ideologies (I like that they help me to get along with more people), but they might be at odds with this kind of pluralism. That would need to be worked out.
3.
A natural answer is to say that there can be "gains from trade", and that not everyone needs to live in the same way. That there can be both cities and rural areas. But,
a. I am doubtful of permaculture's ability to create the kind of surplus needed to sustain urban populations, and
b. we would have to be very careful with the introduction of too much financial intermediation, lest we simply recreate the conditions we now have today.
I'm not sure what the punchline here is, except --
- that issues of political economy need to be factored in here, because they ultimately determine how nature's resources are distributed;
- that power is best when not concentrated; and
- that I'm looking for ways beyond the dichotomies implied by these posts ("dense" vs. "not dense").