well ..
We're pretty much in agreement Johannes, since I'm sure you accept space exploration 'for its own sake', meaning motivated mainly by our best human psychology.
I guess we differ mainly in our emphasis on this:
If this planet becomes uninhabitable to human life, then would it not be the right thing to do; to move and live on another planet?
I really doubt this planet will become uninhabitable, but if it does, yes you're right. But my point is, that if we are using that as an argument NOW, then we're really admitting defeat - that we couldn't convince people and ourselves why we REALLY want to get out there.
(See my post to Raoul on the Space board for more ..)