except for China, Cuba, Venezuela, North Korea, and many African countries

It has definitely been shown. Have you ever seen a capitalist country (which socdem countries are ultimately) that relies entirely on its own local resources. One that does not do business with exploitative global corporations? You can't find one because it doesn't exist and can't exist. Capitalism relies on constant growth and you cannot constantly grow when your resources and labor force are limited so they find more resources and labor forces elsewhere eventually.

Think of it like this, if a business does not grow it stock value stays the same and investors gain nothing. So the business must grow or investors might pull out or invest in the competition that is growing causing the business to fail. So when a business cannot grow because it lacks the local resources and labor force to do so it must find it somewhere else. Usually this happens when a business realizes it can profit more of it finds cheaper (more exploitable) labor from more desperate people in order to outcompete competition. It is inherent to capitalism that it must expand it must grow or it fails.

So from this you can extrapolate that a capitalist country run within its own means will eventually stagnate and either give into the capitalist push to expand beyond its means or economically collapse.

Classic leftist wall of text I know but these things are hard to explain in simple terms without making them inaccurate in some way or making them too unclear.

Related but a bit off topic for those who are interested:

Socdem countries are also doomed eventually stop being socdem at some point. There's this thing big capitalist do where when the government does something they don't like they'll just stop investing in things as a form of protest causing huge economic dips. Eventually as these capitalist under a socdem society realize their profit margins are thinning and they have less to gain from exploiting the global poor they will look for more local ways to increase profit such as repealing workers rights, crushing unions, and involving themselves in government because it's always cheaper to change the law than it is to change the business.

Eventually all socdem countries will reach a point where their capitalist will either leave because they can profit more elsewhere or threaten to leave as leverage for "making business more profitable" by throwing away everything that makes a socdem country a socdem country. The reason this happens is because capitalist will always have a motive to reduce workers rights as well as the privatization of government works such as healthcare in order to profit more. It is inherent to the system