How do you answer SSPX and sedevacantists who cite St. Athanasius to justify schism?

Athanasius did not consecrate bishops. He upheld the true doctrine against error from within the Church; that is the difference. I have always said to the SSPX and the other groups: if you believe you have legitimate grievances against the Church's morals and doctrine, and you are correct that they have made some big blunders, then form the largest, most bothersome, most effective counter-attack you can, but stay within the Church. That is what St. Athanasius did. It was a mess, but it eventually got cleared away and things went back to normal. The first mistake was Archbishop Lefebvre's consecration, an emotional reaction to John Paul II's Assisi interreligious prayer meetings in 1986, which were an abomination. But he was the pope, and the last person who holds the buck is Jesus. If Jesus allows a pope to do these things, you and I have nothing to say except that we do not like it. You remain in the Church, obey as much as possible, and when it is impossible you say, this is where I draw the line. But Athanasius did not try to make a new church with new bishops, and that is the difference.