Don't lock this up! I still have more ideas to share, in my arrogant, YM ignoring way!
It's actually 2 issues. A city, by definition, must be able to house its members. Not just the high-earning people, but ordinary people as well. That would SEEM to mean housing prices are tied to the pay of ordinary people. But that assumes over time ordinary people live in the same type of dwelling. Practical experience in pretty much every major city in the world suggests that as property gets more expensive, the type of property an ordinary person lives in CHANGES. So 100 years ago an ordinary person could have lived in a terrace in Paddington. Today, it's a unit 45 minutes from the city.
When I lived in Hong Kong 20+ years ago, a small unit was about 600 square feet. New buildings in Tsing Yi island built in recent years are anthills with <400 square feet apartments. So 600+ sq feet apartments have grown faster than ordinary incomes to the point where no-one on an ordinary income can afford one, so they downgrade their expectations to a 400 sq foot place. I think they're horrible, but people live in them. Why? They have no choice: if they want to buy, it'll have to be one of those.
In Japan the 'starter' cheap unit is about 25 sq meters (including bathroom and kitchenette). Not in the city, in the suburbs. I used to live in one 20 minutes walk from the station, and the station was 30 minutes from the centre of Tokyo (though Tokyo is so big it's hard to tell where the centre is).
If you built one of those in Australia, would anyone live there? No. Why? Because most people can still afford better. They wouldn't live in a 25 sqm place unless they couldn't afford anything bigger, and THAT is the key to this whole argument. When people can no longer afford one property type, it doesn't mean the price of that property type peaks. Assuming the economic and population growth is still there, it means what people define as an 'acceptable' dwelling changes. No one on Manhattan or Hong Kong or Tokyo expects to live on a quarter acre block, and in fact the expected 'normal' property size is falling in those places. I'm sure people in London would prefer a quarter acre block. Why settle for a 1 bed flat on a busy high street? Because they can't afford anything else.
I also lived in a 40 square meter place in Tokyo, a 60 square meter place in London, and now I live in a 3 bed townhouse, probably about 100+ sqm, in Sydney. Why the difference? Because I can afford to live in a bigger place in Sydney. I simply couldn't afford to live in a 3 bed townhouse in London. Much as I liked to: the price and rent had simply grown far above my salary. So what did I do? Decrease my expectations and rented a 1 1/2 bed flat.
Basically, can the price of your standard Australian 3 bed house on a quarter acre block keep rising faster than incomes? YM doesn't think so because that means at some point ordinary people can no longer afford a 3 bed house on a quarter acre block. But what if, like other major cities, ordinary people resign to the fact that they CAN'T afford a house and so live in units, and houses on quarter acre blocks become, then, only affordable by the wealthy? If you continue to have population growth and increasing numbers of wealthy (and ordinary people), this can happen.
There is a difference between ordinary people becoming unable to afford a SPECIFIC type of property (perfectly possible, and is proven in most major cities) and ordinary people not being able to afford ANY property (which isn't possible, because a city can't survive without ordinary people). Though at that point a city usually keeps expanding outwards. So while no ordinary person can afford a quarter acre block within 15 kms of Sydney's CBD like they might have a few generations ago, Sydney is also a lot bigger now.
Given that our 'standard' city dwelling is supposedly a quarter acre block within 25 km of the city or a 2 bedder within 10 km of the city (or whatever the expectation is these days), and loking at what a 'standard' dwelling is in Tokyo, New York, London and Hong Kong (to name 4 cities that I actually know fairly well), if the population and economic growth continues Australia has a LOT more growth left in it.
All of the above just explains how the price of a SPECIFIC property can keep going up long term. There will be recessions, and booms often overshoot. If you can time it perfectly, you'll do better than just buy and hold. I'm not that confident about my skills, though.
Alex