Paul says that earthly Jerusalem, the Jerusalem which existed in his day, is a picture of bondage corresponding to the state of Hagar. This would have surprised the Galatians and outraged the Judaizers.
Some do not like to have Paul teach that Jerusalem on earth could only ever be in bondage even at its best, and so understand him to say that at the time he wrote, things had fallen to a low ebb, and that it was only the Jerusalem of his day that was a representation of the covenant of bondage. But Paul is making a much more radicle point than this. He is saying that the earthly Jerusalem could never rise any higher.