As has been remarked, the problem is less about extracting it and more about burning it. However, to answer your question, in the absence of any extraction its

As has been remarked, the problem is less about extracting it and more about burning it. However, to answer your question, in the absence of any extraction its still quite easy to make oil from any carbon-based substance (search "thermal depolymerization"). We will never run out of oil - at worst, we will run out of "cheap" oil. However, as plastic is ludicrously cheap anyway, I don't see that as a major problem. Sure, it might cause some companies some headaches, but it won't cause any major disruption to modern life.