First of all, let me congratulate you for a well written post. Then, do you really think the idea to explore the possibility of human survival in order planet stemmed from the belief that the earth has a expiry date? Granted that it is truism that the earth has expiry date due to our destructive nature, I really do not think moving into anothe planet will save us. 

However, I really don't want to believe scientists are exploring this possibility out of mere fear that the earth might end up being destructed by and large our actions. There are practical measures that can be put in place to at least reverse the destruction, mostly by emitting less pollution and taking care of the environment generally. Moving to other planet is really not an option for the human population.

Lastly, I do not believe that the world is overpopulated. The earth can sustain more population that it is currently holding and the idea of overpopulation is just a political propaganda as far as I am concerned. The main problem that has been pointed out is inadequate resources, the same resources some people are hoarding for unborn generations. Greed is the game, not overpopulation and moving to another planet will never solve our problems. It will probably complicates them