To me, this is one of the most interesting elements of a whole bunch of economic myths, including Ayn Rand's frontier style libertarianism. Eternal growth (and the general "morality of self-sufficiency" not to mention "if you're poor, it's because you're lazy") all powerfully run off the notion that if someone "eats your lunch" you just run off to "new territory" and do something new.

A basic truth observing large parts of the world is that a lot of the most productive, most habitable land has already been exploited, but our economics sort of lives by the idea that such things are in infinte supply.