They're not. Downsizing is necessary if they want to live in Toronto. Nobody needs to live in giant McMansions. Alternatively, working in their suburb (such as from home or from an office there) wouldn't require them to move at all.
Why? I can afford to do it. And I'll continue to do so as long as I can afford it.
An individual choosing to not ride a motorcycle or drive their Ferrari for pleasure won't make any difference at all to the environment. But millions of people who suddenly can't afford their luxuries (such as recreational vehicles, gasoline, and large houses/long commutes) who are forced to stop buying/consuming will make a massive difference.
Edit: reworded a couple of sentences.
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