Other than natural demand, Australia has a high real estate market due to the tax and a superannuation/pension distortions. Should try to fix those first. (probably impossible)
Let's put a number on it. Since the article uses $400k as a reference point, let's use that. You could afford to buy a $400k house back when you bought your current house. You cannot afford to buy a $400k house today. That would be true whether or not you had purchased your current house, and regardless of the interest rate on its mortgage if you had.
You only "can't afford to sell your house" if you're underwater on the mortgage and can't come up with the money to sell it.
We feel trapped because we would have to massively downgrade to move. Obviously, we COULD do that, but we don't want to.
You are right, we also couldn't afford to buy our current house if we currently didn't own a home, either. I am arguing that the fixed thirty year mortgages artificially drives up prices, which means that you are stuck and can't move whenever interest rates are high.
If we didn't have the fixed thirty year mortgages, housing prices would have never gotten so high, and buying and selling houses would be a lot easier, and people could move to where they want to be much easier.
Whether any specific person actually thinks through whether spending as much money as the bank will lend is prudent, instead of buying a house they can actually afford, saving the money, and upgrading later, is a different question. But it's not fair to blame the mortgage itself.
Anyone who ends up with their primary residence as their "biggest investment" has been a piss poor investor.