There's no "day oil ends"
it's just going to fade away very very slowly by becoming less and less useful, less economically interesting to use vs alternatives.
For plastics, it will always be used, but as a transport and heating fuel, where the majority ends up today, it'll go down, and the price is gonna be very volatile when demand and supply adjusts each other.
The big question is: how will the governments compensate the loss of transportation fuel taxes ?
http://industry.eiu.com/asset_images/1622440146.gifMany countries have over 50% tax on fuel, and that can represent a very significant part of a country's GDP...
That fuel tax is currently not applied on airplane fuel, heating fuel, offroad and agricultural fuel, plastics,...
It'll probably have to partially shift to those other uses of oil.