There are no mines with unlimited resources in the ground.
most oil wells in the southern US are all empty now.
the reserves in the ground are always limited.
in Simcountry however, you know exactly how much is left in the ground.
These corporations run for many real months and they are extremely profitable. Bu the time they close, their owners have made at list many hundreds of Billions.
It is important, also in Simcountry, that at some point they have to close and require you to find a new resource.