I think that's funny. The essence of the sin of schism is refusing to properly submit to the person recognized to be the Roman Pontiff.
It is true that being mistaken about the identity of the Roman Pontiff does not by itself constitute schism (which pre-Vatican II theologians have held, and which is my considered opinion as well), but the difficulty is that this was only often granted by both sides practically in Church history when there were two or more visible claimants to the Petrine See, which is not currently the case. For example, St.Vincent Ferrer and St.Catherine of Sienna were found on different sides during the deplorable confusion of their day.
But if one can personally decide a succession of Popes are simply antiPopes with no Cardinals remaining, no identifiable visibility of the Church enduring, with no end in sight, frankly, I don't know. Where does it end?
The situation is exponentially more complicated and requires all sorts of things, including the divine promise, the indefectibility of the Roman Church, the visibility of the Church and several other dogmas to be taken account of.
This is why I think the Archbishop considered it imprudent and at best an uncertain or speculative position to be tolerated. As for the SSPX, epikeia and the doctrine of necessity is well founded. But even more than that, the 1983 code which would currently apply is so lax that one could argue, as Michael Davies I think successfully does, that the charge of schism cannot be made to stick.
Why try to represent what he thought in 1979 when 7 years later (1986) he most clearly and publicly changed his mind?
Because all the reasons he offered for thinking so, ("The visibility of the Church is too necessary to its existence for it to be possible that God would allow that visibility to disappear for decades ... Who will tell us who the future pope is to be? How, as there are no cardinals, is he to be chosen?" all held as true in 1979 as they did in 1986 as they did after the deplorable events of Assisi, as they did after the Econe consecrations, as they did after the fallout with Rome, as they did at his death in 1991, as they do today and still "puts the Church into an inextricable situation"
Perhaps, Cupertino, you should consider that the Archbishop heard all the reasons you offer for sedevacantism and rejected them for reasons that you have not considered. Those closest to him can vouch that he never changed his mind inspite of, humanly speaking, being given every reason by his shoddy treatment, in favor of doing so.
Cardinal Ratzinger is reported to have said that among the two regrets of his life was what happened between Rome and the Society. Though I don't agree with the Archbishop on every count, inclining myself to the FSSP view, I believe as he did, and as the Society believes, that the difficulties started from Rome and the solution will come from Rome. And pretty much everyone whether FSSP or SSPV or one who is grateful for the Indult/Motu owes a debt of gratitude to them.
And whatever anyone may think, if Tradition flourishes today worldwide at least as much as it does, if Rome has recognized that the traditional Lass Mass was never abrogated, if today every priest is at least free
de jure to celebrate the Tridentine form, then the glory redounds in the highest measure to the Society of Saint Pius X.