We've got to define "unlivable". What if statistically, everyone's house will be destroyed by flooding, forest fires, etc.. let's say, twice in their lifetime. How can the math possibly add up if a house's price requires a 50 years mortgage? At some point, something's gotta give and that kind of odds will happen in a timescale shorter than many living people care about.

Also, wasn't there something about, you know, caring for the generations to come? Maybe I misremember...