> why would you think 'modern civilization' would be limited to one planet?

Because we're nowhere near ready to live outside earth! Approximately no human lives in a desert, on a glacier, or below the ocean surface, even though that's easy compared to living anywhere else in the solar system. Humans are not made for space.

My prediction: By the time we have sufficiently advanced tech to keep humans alive in space, the humans won't be necessary anymore. Why ship delicate bags of water between planets? It's sheer hubris to think humans will play a role in space. It'll be machines that don't suffocate or freeze at the slightest inconvenience.

Well, we haven't begun to populate any other planet yet and time does appear to be running out in terms of us seeing the effects of such a huge mass of humans. Getting a population started on another planet is going to require a LOT of investment of both money and natural resources and also it'll involve a mind-boggling amount of energy to get enough equipment out of our gravity well.