I got an interesting question from a user with regard to the effect of moving all those players back to their original nation. In essence, the question comes down to, does moving them back, make those nations better or does it make the nation worse off.
There are two things to take into consideration.
The effect of playing high level football
FM
As the Premier League is the place where most if not all high level players are sucked into, I start there. Foreign players playing in the PL will get better as they get better resistance which means they are grow with that or hit their ceiling. So opening up the borders meant players that otherwise would hit a (probably) lower ceiling in their home country can continue growing. At the same time foreign players will take English players jobs. The effect isn't that strong seeing the general level of England the last few years, but Maguire sitting on the bench at Man U, will not make him any better. I don't know if it has been researched, but I would expect a nation where the export vs import of foreign players will see a plus in development if the export goes to mostly superior nations as it is a developerhelper.
FM has a funny role here. We don't change player values, but change a few nation values. FM will react to nation performances, so a boosted Suriname squad with players like Van Dijk and Wijnaldum will improve immensely. Because players like Lewandowski have profited from getting better in Germany, Poland will profit as well. We can't say if he would have been as good when he would played in Poland for a while longer, but that's in the end not our goal.
Anyway, that's my slightly more than 2,5 cents for today. Back to the Soviet Union!