My proposal tries to take that into account by both acknowledging the facts on the ground and not redrawing any official lines.
Most Syrian Kurds and Sunni Arabs do not want to be ruled by the Assad government in Damascus. There have been multiple Sunni rebellions in Syria in recent decades, and forcing all of them back under Assad’s control would likely lead to another in the future. And that could easily lead to an ISIS successor.
However, creating new countries would redraw lines, which could lead to numerous problems. The Syrian Kurds and Sunni Arabs might like it, but Syria, Turkey, Iran, and probably Russia would not. It would risk regional war.
Therefore, semi-autonomous regions is the least bad of many problematic options.