That argument can lead to the assumption that it's ok if we completely wiped out life on earth as long as humanity survives by leaving for the stars.

The earth is the only place in the universe right now where humans can survive at our current technology level, and we are destroying that ecosystem at an alarming rate. We depend on the entire planets ecosystem for our survival, and even if technology somehow could let us survive ecological collapse, a humanist approach to the environment is tantamount to burning the boat we are sailing on to keep warm.