That only ensures that flying cars will always be more expensive than ground cars, not that per capita income will always be too low to have mass adoption of flying cars.
True, but the economics of energy/housing/transit indicate that there never will be mass adoption if it's also the more expensive option.
Cost isn't a big deal when it's less than a 50% increase, we're talking 3-5x here. This is equivalent not to a Hummer but to a Ferrari, that's a difference in sale volume that makes a flying car unit unsustainable give the increased non-traditional expenses.
That depends largely on any negative value that's charged against living in an undesirable location, and on the subjective value a person ascribes to living in a desired location. And that subjective value can remain constant, while the cost of energy and transit decrease, and the economic value created by the person increases.
Yes, it would probably be economically more efficient to live in low-cost housing projects within a few minutes walk of the offices of megacorporations. But millions of people (76% of Americans, according to this 2009 survey: http://www.census.gov/prod/2011pubs/acs-15.pdf) choose to drive an single-occupant automobile to work, averaging a 25-minute commute each way. And few would give up their acre of land and private home to live in an apartment closer to work.
If you could take a high-speed helicopter from Monterey to an office in Silicon Valley for $5 and a 15-minute flight, instead of hours of your time, or $500 in helicopter costs, would you really live in on-campus housing because it was more economically efficient? Yes, flying a helicopter for an hour takes a lot of energy: on the order of 1,000 kilowatts per hour. And yes, helicopters are really expensive and niche. But technology is constantly decreasing both the cost of that energy and the difficulty and operating expenses of flying machines.
1000 kW, or 1000 kWh per h. That's the beauty of P = Power = W/t = Energy / Time.
There are two things I feel you neglect:
a) These increases happen in parallel, meaning that as air vehicles get better so too do ground vehicles. Meaning it's unlikely for the relative advantage of cars to change with technology improvements.
b) I can state the last sentence because of the physics and engineering fundamentals that go into ground and air vehicles. There are hard and fast limits to the physical world, air vehicles have to butt up against a lot more of them than ground vehicles do.