Although coal and petroleum are produced by degradatioin of bio- mass, yet we need to conserve them. Why?

Production of coal and petroleum by degradation of biomass is a natural process. Although it is natural, it takes several million years to be completed. If these fossil fuels are used in excess, then it will lead to their unavailability. Hence, future generations will not be able to use these fuels. Hence, sustainable use of fossil fuels is necessary.