In any reasonable discussion about economics, I would assume that "until they're gone" means "no longer profitable to exploit" as we're all aware that short of

In any reasonable discussion about economics, I would assume that "until they're gone" means "no longer profitable to exploit" as we're all aware that short of extinct species that are beyond our technological capability to recover, nothing is truly "gone" as long as there is someone capable of expending gargantuan amounts of money and tangible resources flying to another solar system (or galaxy or galactic supercluster) where those elements are still easily available. Really, then, it's a function of universal state. Regardless, here on Earth, eventually we will either reach a point where growth in energy production is practically infinite or we will be faced with the rather herculean task of converting all of our waste back into usable material, ad inifintum, with less energy available than was put into the system to create that usable material to begin with. At that point entropy comes knocking on the door.