At no point did I assume that. If, I extrapolate based on something that is not true (aka we stop mining uranium) and look at it in a given area (aka the land area currently occupied by the USA or the Earth) then clearly I am not trying to predict human behavior in some other location.

If you want a prediction of human behavior, fine. I suspect that we will move to a mixture of fusion, wind, and solar power long before our supply’s of fissile materials become an issue. I also expect that our initial expansion into space will primarily involve exporting technology into space and collecting raw materials from space. However, initial space exploration is going to have minimal impact on life on earth with few material goods being sent back and forth. I also suspect that it’s going to take far longer than 1000 years before 1 billion people live outside the earths atmosphere.

Edit: The Americas where "discovered" 518 years ago and dispite being there for the taking and far more hospitable than space 200 million more people live in India than all of the Americas put together.