The problem with this description is that neither long-term demand nor supply is fixed but depends on long-term expected price.

In normal usage in economics the "long-term" refers to an equilibrium where supply matches demand. Demand never outstrips supply but instead drives up prices until a new equilibrium is reached at a lower quantity and some current petroleum users are forced into alternatives.

This is the essence of economics: the study of the allocation of scarce resources. Notice "study" as this is a completely natural process.