given that there is quite a bit of land, if the population naturally moved up to 1 trillion, I don't think housing would be more expensive.

You might argue that "naturally" is doing a lot of work here.

I will concede that if the population changed dramatically in a very short period of time, we would have distortions of price. But immigration has been an ongoing phenomenon over the last several centuries and I don't believe that the market has been unable to adjust to it.