Pretty much every physical object we had started out underground and was mined by human or plant activity, except for the minerals that are synthesised directly from the air.
It isn't a hill I'm going to fight on, but 'land is a finite resource' has, to me, always meant arable farmland and prime coastal real estate. There is no actual shortage of land. At best, there might be a shortage of prime away-from-water-table-with-less-permeable-rock-surroundings sites that are ideal for landfill - but I doubt that is true.
For example, the deepest mine is about 4km deep, and about half the atmosphere is within 5km of the surface of the earth. Running out of landfill space in that sense is like running out of oxygen. The only limits are transport costs and making sure the landfill is either non-toxic or kept well away from water (which, economically speaking, might be a substantial limit but plastics are clearly quite inert, because we store food in them and they apparently don't degrade).
EDIT But the key point here is that these costs aren't externalities. The people paying for them are the consumers who are buying the plastic. If landfill costs go up, municipalities will start to charge more for waste disposal and consumers will favour products with less packaging.
