There are other sources of energy. Energy efficiency will continue to improve. We will find ways to harness solar energy that do not require us to pave the world with panels. Society will re-examine its strange phobia about nuclear fission. We may develop fusion.
Fuel cells are likely to become efficient and cheap.
To sum up, the title of this panel suggests that many natural resources are declining and non-renewable, and that society's task is to manage scarcity. I can think of none that meet these criteria, if we focus on the purpose served by a resource rather than on the resource itself. Given the power of price signals, the extensive potential for substitution, and the accelerating wonders of human intellectual capacity, it's hard to imagine ever hearing of one. I may not fear scarcity, but there are some real hobgoblins out there. Its end point could logically lead to a case for resources, so as to preserve them for future generations.