Sydney is a boom bust market.
To test your theory lets do the math. Let use a couple on $150k (lets say 75k each). After taxes they will key about $115k. Lets say they buy a 650k home with a loan of 600k over 30 yrs.
At 5% they will pay about 36k in interest at 7% they will pay about 48k in interest.
Their disposable income crunches down from 79k to 57k that is a lot if disposable income. A lot of people would be going backwards. Pay rises over 2 years for a couple would be 10k but after tax they will only be left with 6k. So there is still a short fall and more than half of this would be taken by inflation.
So the opportunity cost of not managing is a bigger risk.
On an investment site you talk about a couple with a PPOR and refer that to opportunity cost??? What about someone with multiple properties, yes the costs are increased as the debt would be larger, but would likely come with a combination of increase rent and CG.
If no purchase of IP is made, there is opportunity cost as no CG made, and the ability to leverage is a whole another level.