I have my issues with Sedevacantism, but not when it comes to rejection of the false popes of the Conciliar Church. Once a pope espouses and teaches a heresy, he ceases to be pope de jure; he remains in the seat until removed; that's simply a fact, of course (he's sitting there). But his authority is gone, and he has no claim to obedience. He becomes anathema by virtue of his denial of the "doctrine of Christ" and promotion of an "anti-gospel."
Biblically supported: Galatians 1:8,9; 3 John 9-11.
Magisterially supported: Paul IV, cuм ex Apostolatus Officio.
Sedes aren't the cause of disunity. The excommunication of Archbishop Lefebvre had nothing to do with Sedevacantism: they want their schism, and they want you to eat it too.