That's a logical premise. The debate can never be settled anyway so this is one suitable solution to trying to make sense of the issue. Thank you, Brian.There are probably other things to consider. God promised exile for the Jews if they were not faithful to the Covenant and during exile there could be no King in Israel or Judah. So prophecies about the Messiah needed to fit with that promise also.
Maybe the best way to understand it is that there would always be someone from the line of David who could become King and that all the Kings of Judah would be from that line.