At $20 or $30 a gallon, oil is completely unaffordable as a fuel but still perfectly viable as a raw material. Obviously we'll substitute out other materials for the least important use-cases, but we won't have to radically rethink the manufactured world. According to the DoE, plastic production accounted for just 4.6% of petroleum consumption and 1.5% of natural gas consumption in the US.
The costs of all raw materials are rising, many of them much faster than the price of oil. Many metals have doubled or tripled in price over the past few years. Commodity foodstuffs have surged in price. Quality hardwood is increasingly scarce. We fetishise oil, but we have a generalised resource problem due to the rapid improvements of living standards in the developing world. There are only three plausible scenarios - people getting used to consuming much less, the development of nuclear fusion, or a third world war.