Well, Namibia suffers from the "dutch effect." This is also true of The Saudis and others like them. These are circumstances where a single hugely valuable export strengthens the currency and depresses the development of other domestic industries. (Namibia has a million other problems too.)
Your example of the scandinavians is interesting in that the North Shore deposits made them initially fear this very result. But these are certainly *not* vibrant economies though I admire the industriious character of the populations historically.
I would ask that you compare a Sweden with a few million people to an America (75 times bigger in population) which doubled in population twice since 1900 and still provided jobs, high living standards, the lion's share of all world innovations etc.
Generally speaking, when the rich are getting richer the poor are also getting richer. It is not the gap that matters but the sheer amount of wealth produced.
The biggest problem in socialist logic is the idea that wealth is a pie. Cut one piece bigger and the rest must be smaller. This simply isn't true. But the falacious nature of it was more obvious when many Americans lived on farms and knew that their own wealth was not diminished just because a neighbor cut trees to build a house or cleared land to plant a garden. ***My neighbor's wealth does not make me poor just as his intelligence does not make me stupid.***