If you are a permanent resident of another country, as I am, you are at their mercy--again, as I am. Furthermore, all of my income is in the USA, so the Germans may not understand all of the mandatory taxation that some income is subject to (or choose not to).
Every year, they ask me if I have taken any distribution from my Roth IRA (I have not). I made the Roth conversion before I moved my residence here. That meant I paid my taxes on it up front at the time of conversion. Under US law, my tax obligation was paid in full. The Germans are hinting that they now want 50% of that, too, even though the double taxation treaty clearly states they are entitled to none of it. The treaty says that a US citizen resident in the USA, who is taxed on funds in the USA, cannot be taxed again on those same funds by Germany. Except that the Germans are saying, well we want 50% of it anyway, never mind that you paid your US taxes on it already.
Maybe they want me busking on the streets of Düsseldorf or searching for deposit return bottles in the trash at train stations, like so many Germans are already doing. Not everyone shares in the relative prosperity here. My wife is a retired German social worker, and worked all her professional life with people here who fell through the cracks. Their number is greater than most people imagine (or prefer to admit).