On the contrary, if we flatline our population at a low enough level, we can maintain a high tech society indefinitely on this planet. The only materials we are truly consuming are uranium and other materials that we transmute to other elements. With enough energy input, we can recycle *everything* else. We can even take CO2 out of the air and turn it back into coal if we want.
It's simply a question of managing our resources for the long term.
And humans can do this, there was an isolated island in the pacific which maintained a good standard of living for hundreds of years via limiting population and managing resources until they were interfered with by outsiders. Their means of population control wasn't pretty--infanticide. However, we have better ways now to control population and in principle we could do the same planetwide.
Another example, the Japanese have re-forested their island, another example where humans can maintain and improve their environment, perhaps indefinitely. There's no need for the "herd" to move on if the "herd" maintains a good environment.
Just because humans presently are mostly NOT doing this does not mean we cannot.
Though I would prefer that humans self-modify so that they are more suitable for space habitats and move off the planet. The planet is only sustainable so long as there's no really big cataclysm of whatever sort.
So I agree with your point about colonization, however, I do NOT agree that 'using up the local resources' is the driving reason for diversifying habitat.