Call me stupid (I am not an economist) but I can't see how your conclusions follow from one another.

> That implies that that 5 million more people are now earning Boston-level salaries and 5 million more people were able to make the personal choice to move to Boston.

...How did those 5 million better-paid jobs got created? How did the 5 million people connect with them? Not everyone is 18-25 y/o you know, a lot of people know one profession and don't want to ever learn another (assuming the jobs pertain to a different profession, but I'd say that's a safe bet that not all of them will cover the professions of these 5 million people).

> These 5 million people are on average doing higher-productivity jobs than before, which means economic growth.

Better salary rarely correlates with better productivity in my experience and observations. People love their titles and prestige and are chasing after them just for the extra salaries they bring. And even if we ignore that, a lot of companies pad budgets for well-known reasons: if you don't spend your entire department's budget then it will be reduced in the next quarter or fiscal year, and you as a manager are very likely to get skipped when handing out bonuses. This has often led to managers hiring expensive consultants that barely do anything. These people are laughing their way to the bank and the economy gains nothing from it (those consultants will not reinvest 100% of the money in the local economy; I'd count it as a huge win if they reinvest even 20% but I have no data proving that they do or that they don't).

So yes, while the new 5 million people could have better salaries, that says almost nothing about productivity or helping the economy at large with this theorized increased productivity.

> Yet at the same time, these 5 million people now have a lower ecological footprint than they would have had if they stayed where they lived before.

I am most of all challenging this one. I've known a lot of people in rural areas years ago, some of them pile wood and small amounts of coal for 3-5 years ahead for prices I'd find laughable and a rounding error in my 3-month expense budget -- and before we talk about ecological impact or CO2 footprint, most of this wood was cut 5 years ago, it's not removing new forests. They manage their heating extremely efficiently. It really opened my eyes how much us the city people look down on others in terms of ecology when in fact we know next to nothing about how is our local electricity procured, how much CO2 it releases in the atmosphere, and how is it even subsidized (a huge topic for another thread / time).

So obviously this is down to context (and I have no idea where does Boston get its electricity, or any such city that your idea might pertain to) but to me this is not as clear and cut as you make it sound.

...And that's not even mentioning rural people getting solar panels and literally having 5-10% of their winter power bill during the summer.

......And it's also not mentioning that some of them did a combination of a family picnic + were gathering dead wood / twigs / dry leaves sometimes and these easily accounted for no less than 5% of all their heating in the cold months. This is not destroying the ecological system, it's merely being super efficient by removing mostly useless bio matter (and very far from such a huge scale that it will hamper enriching the soil, before you say it!).

> These 5 million people are arguably better off, and the rest of society is also better off.

As you yourself alluded to, that's not a given, like at all. 5 million people moving away from rural areas could arguably leave 10+ million elderly people without care or even ability to have their kids arrive to their location quickly if they have a medical emergency.

Which part of "society" is better off? City real estate renters for sure. Who else?

Local sandwich shops, laundromats, coffee joints, ice-cream truck? I'll give you all of those and I like how city people inter-mingle both in terms of physical presence and capital. But I remain skeptical that society as a huge entity is actually better off.

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IMO your take is a very idealistic one. Still, I do my best not to sound like I am attacking you, I am kind of confused that I can't see a single correlation in your string of assertions and I am curious if you can back them up with something more than aggregated (and very diluted) historical statistics.