You might not be restricted to five main networks, but that doesn't mean that everyone makes use of the added variety. Choice overload is a thing. Most people that watched FTA TV yesterday won't consider streaming today. Most people that watch FTA today won't consider streaming tomorrow. They're going to pick their entertainment from the same pool of choices they have.
I don't have data for Australia, but if you don't mind using Google Translate, i can show you data from Europe that shows that people tend to pick their entertainment form a limited pool of options. But you might also check and see if Big Brother drew a huge number of young people back to FTA or if it's mainly the ones that already watched FTA last year but now switch to Big Brother instead of watching whatever they've watched last year.
It's obviously not true for every single person, but it's true for a bigger group of people.