The day we need more resources, you can be sure we will go and get them where they are. We have not have had the need to dig deep or explore the oceans so far (because other resources were readily available), but there is no telling we will never do so.

> or oil, which has been getting harder and harder to extract.

Harder to extract but still in much larger abundance than what was predicted by mostly everyone back in the 70-80s. I don't know if you were around then, but you may have heard of "peak oil". It never happened (as in, not yet, and that was already a long time ago).