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.