--As the legacy petrochemical supplies become less available, their cost will increase.
--At some price level (appx equal to current petroleum prices, in Brazil) the cost of producing fuel from fermenting weeds or pommes frittes or methane-rich cows will equal the cost of equivalent fossil fuel. This will dramatically reduce the demand for fossil fuel futures and force sharp breaks in price. Supply demand will oscillate for a century or more, but 'free-market' economics will eventually lubricate a relatively smooth transition from Petroleum to the Panopoly of alternate fuels and energy sources, even as global energy demands increase.
--What the oil Cassandras are seeing now is really just a cloud, not the sky falling.
--If we can clean up our wasteful uses of energy in the process, that would not be so bad, tho. Unfortunately, travel for 'entertainment' might be one of the first wasteful uses to come under pressure.