I think a lot of the desire to become a two planet species dwindled at the close of the cold war. Without the constant threat of nuclear annihilation it just didn't seem as important to people. I agree that we are going to need to diversify if we want human life to keep going, Planets and solar systems. I wonder what will happen when AI gets to the point of being self realized, it could expand to reaches that just aren't in the picture right now for humans.
I couldn't agree more with both of your main points (i.e., cold war, AI). To be honest, I've written before that we won't really be able to colonize other planets, and certainly won't be able to explore the rest of the galaxy unless we experience a technological singularity. Since that will likely happen between 2040-2070, I wouldn't be surprised if we really started to establish ourselves within the galaxy in the coming centuries.