Curiouscatholic,
Benedict XVI thinking he is a "conservative"
Catholic faith is not in "conservatism" but in the divine promise, that the Faith of Peter will not fail. I believe, as do some others, that we are in the hour of the Passion of Holy Church, and therefore, it proves unsurprising if Peter unfortunately turns out to be rather weak, as he was then. But as we do not assert that St.Peter became a "formal apostate" or "notorious heretic" after his denial of his Friend and Master, so too we believe that Our Lord will strengthen the Pope, as He did the Apostle, in His own good time.
The difficulty for those who espouse the sedevacantist thesis, is to establish the two premises on which their argument is based.
1. That all Catholics are bound to become sedevacantists in the event of a Pope who appears to have fallen into heresy.
2. That the Pope has actually appeared to have fallen into heresy.
And despite the focus on 2, it is really the minor premise of this thesis. The focus should be on premise 1. Also, it goes without saying that no amount of apparent proof of 2 counts for anything much without 2 being proven.
I wonder if you believe Honorius was a heretic? I know some deny it, and it is not impermissible to do so, but at least for those who do not, it seems to me, there is a watertight historical example contra-sedevacantism. And this is St.Maximus the Confessor, Doctor of the Church, "hammer" of the monothelites. St.Maximus vehemently opposed the monothelite heretics, but after he brilliantly exposed their sophistry, when Honorius was cited to him by his opponents trying to prove their heresy, he defended the then Roman Pontiff from the charge of teaching heresy. He did not become a sedevacantist.
Clearly, this is an instance of him subjecting judgment to the Church and not presuming to arrogate it to himself. And this is perhaps the only publicized incident in Church history where in all probability the Pontiff in question had actually succuмbed to heresy. Even here it was left for the Church, not private individuals, to decide. So here we have so great a Saint who did not feel so confident on the one hand to judge the matter rashly and on the other, dogmatic sedevacantists who wish to tell us their position is so obvious and is binding on all faithful Catholics as if it were an object of Faith necessary for salvation. It seems to me we should at least tread cautiously in taking a conclusion.
For the record, all said, I grant that sedevacantism can be held as a plausible theological position but that is different from 1 as I've stated it above. Sedevacantists, even if mistaken, according to at least some Catholic theologians would not be in schism. However, personally I still believe it would be harmful, not least to their own spiritual life.