> . People would just abuse the system, using highly capitalist, low tax countries during their youth and move to socialist countries as they as getting older (of course that does not work, you need borders for this) and it's utterly unfair to let people from outside abuse a contributive system.

The United States is kind of a micro example. Federal taxes are uniform of course, but state taxes and services are not. Retirees tend not to move to high tax, high social services spending states, they tend to do the opposite: move away from California, New York, etc to Arizona, Florida, etc to live less expensively, even if there's fewer services.