If commodity X cost $A(small), and commodity Y costs $B (large), then there's little incentive to buy commodity Y. But as $A increases, it becomes more worthwhile to buy commodity Y. It's the same reason extracting oil from shale is only worth it for relatively high cost barrels.

Right now the reason we have government subsidies for alternative energies is that they are not economically viable. But as the cost of oil increases, those non-fossil alternatives become more viable. If we were to (hypothetically) run out of oil entirely tomorrow, the price of what had already been pulled out of the ground would skyrocket, and people who couldn't afford it would go all-in on renewables, regardless of the cost.