Germany's tax system is linked to residence, not citizenship (unlike the US). Therefore if you move to the US and work there for a few years, you will not pay any German taxes during that period.
Therefore the "free rider" effect definitely exists, but I don't think it is a huge problem, because the numbers of people migrating out and in again are not that high.
Therefore the "free rider" effect definitely exists, but I don't think it is a huge problem, because the numbers of people migrating out and in again are not that high.