Some important context here: this applies to probably every Australian city but is particularly prevalent in Sydney I feel. And that is that the notion of what Sydney in keeps changing.
So when I say Sydney became unaffordable in the 1970s, I mean what people viewed as Sydney back then, which is inner Sydney, the North Shore and coastal suburbs. But these have now been so out of reach for people who aren't wealthy for so long that people don't think of them as Sydney anymore.
Now when people talk about the affordability of Sydney they're talking about the areas where "normal" people buy. Sydney has expanded far to the West, basically to the Blue Mountains (eg Penrith). It's also expanded south (eg Campelltown). But somewhere like Mosman isn't part of the normal person's mental picture of Sydney property where it was 40+ years ago.
This reminds me of something that happened to me in university. I mentioned to someone the suburbs I'd be staying in over the summer when university was out and they literally said "la di da" like it was fancy. To them it was because they were part of a newer generation who lived very far north (I remember they took 3 buses to get to university). To me, it was just where my grandparents lived and had done so for 40-50 years. When they bought that house it was literally the edge of the city, as in the other side of the street was bushland.
You see it on the train too. When I went to university the one where all the students and twentysomethings lived was 5 stops from the CBD. 15 years later it was 8-9 stops.