The population distribution is as much due to where things already are, and a lack of population, as it is to the climates we’re talking about. If it’s just about climate, why is Las Vegas the 25th largest city in the US? Why do people move there from the coasts or more temperate climates? Australia could absolutely make use of much, much more of its land mass. They just don’t have the population to support it, and they won’t in the near- to mid-term because of their incredibly restrictive immigration policies.
Canada is a much better example to support your point. Large swathes of that country’s land are absolutely inhospitable for most of a year. But that’s going to change as the temperate band moves or expands thanks to global warming. If you’re young enough, buy a cheap chunk of dirty ice next to a permafrost lake, and make bank in 25 years when it’s a tropical beachfront and everyone wants to live there :P