Not long ago I thought the same thing. We can live on the ISS, so even if there's no oxygen and temperatures will be perpetually 200ºC above current, I'm sure we can construct habitats (even small cities) that could be self-sustaining and live. But here's the catch: these things don't happen instantly. It's not a turn-based RPG where somebody decides one turn to "build self-sustaining, hermetically sealed arks", before these things become an absolute necessity we may have already lost the infrastructural or technological abilities to do so (even the raw materials come from /everywhere/ on Earth these days). Worse yet, when the social-economic inequality is the difference between life and death, I'm sure a lot of people will be very angry. And very angry people do stupid things. So you build an ark to live, but not everyone outside is truly dead yet, I'm very sure these people would like to get in. Destructively.