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"The total mass of the asteroid belt is approximately 4% that of the Moon" from Wikipedia. I think you're massively underestimating the sheer quantity of stuff in the Earth. That said, of course physical resources are not literally unlimited - they're just so abundant that the primary limiting factor (by a long way) is our resourcefulness in manipulating them.

"At least not until we can start mining asteroids."

Exactly my point. "Resources" includes every tech you can imagine, noting the actual techs will almost entirely consist of things better than you or I can imagine.