There were no other takers. Because he understood that, strictly speaking, there are no natural resources. Nature provides man with lots of variegated stuff. But stuff is only a potential. To transform it into a resource, a creative person must first stamp it with a human purpose.
People create resources.
The ultimate resource is human intelligence. If individuals are free to think and create, we will have an unending supply of resources. The more people, the more creators. By the same token, for Simon immigration was a good thing. His work on population prompted F.A. Hayek to write a fan letter to Simon, the only such letter he'd ever written to an economist.