I have a citation[0] that disagrees with your characterization, suggesting (the obvious) that while it may technically be possible for humans to live in harsh environments, they tend not to, they tend to live in a small subset of all places, and climate change will move those places everywhere, even in the first world.

"We show that for thousands of years, humans have concentrated in a surprisingly narrow subset of Earth’s available climates, characterized by mean annual temperatures around ∼13 °C. This distribution likely reflects a human temperature niche related to fundamental constraints. We demonstrate that depending on scenarios of population growth and warming, over the coming 50 y, 1 to 3 billion people are projected to be left outside the climate conditions that have served humanity well over the past 6,000 y."

Unless you are cheating by saying 'first world' without noting that the places constituting the first world will likely migrate rapidly in the coming century.

[0] https://www.pnas.org/content/117/21/11350