No one can ever, ever, successfully argue that it is in the 'scope of practice' of any Catholic, even a priest or Bishop (especially without formal theological training or Ordinary Jurisdiction) to determine the status of the papacy. And, my friends, I do not believe that is not without reason.
Sedevecantism may be an opinion, but I believe it is a grave position and opinion to hold, and borders on the schismatic, though I do not yet say it is schismatic. Though, the more I read, the more I find I am being lead to this conclusion. I recognize outright that I am no theologian so I will probably not convince anyone of my views, much less win an argument, nor is that my goal here.
That said, it is that recognition of my humble position with which God has granted me in life which is what originally led me to reject holding such an complex and compounded position, whose implications are inherently scandalous. Sede's have argued that the problem is actually very simple, but that is simply not true. That one can simplify their position, is one thing; even atheists can say their position is simple, but we have all seen the extent required of them to support such a 'simple' position.
With this in mind, I would say the following two points are, for me, the most compelling argument against sedevecantism: (1)for a Catholic, there is no need to make such an 'opinion', which determines the status of the pope or the vacancy of the Chair of St. Peter; (2)I am no theologian, and even discussing such an issue requires one to delve into matters of doctrine and cannon law which are inherently complicated. Though I believe I am of moderate to high intelligence and wisdom, I recognize my fallibility, and in doing so I also recognize that the latter point, which builds upon the former, can lead me to a position which is dangerous and, thus, potentially damnable for my soul.
So while the above points allow all Catholics to remain free of the sedevecantist position, and thus a fallible opinion (which I believe is inherently dangerous, as proven by many dogmatic sedes) another point which I have been contemplating more recently is the perpetuity of popes promised by Vatican I. And while I currently believe that sedevecantists are entitled to their opinion, and I my own, I am starting to come to the opposite conclusion; that sedes may 'not' be entitled to such an opinion, and that it may even be schismatic. As I recognize above, the issue is complicated, and I am not learned; but if an sede says that they are merely holding an opinion, here is my own:
Though I've seen Fr. Chekada's arguments explaining 'why' Vatican I made such a statement, which he says, and I believe, was directed at the Eastern Schismatics, those statements are still an argument against sedevecantism, despite whatever the reason which prompted the Council Fathers to make such an dogmatic statement; it stands on its own: "Thus, whosoever succeeds Peter in this Chair, obtains, by the institution of Christ Himself, the Primacy of Peter over the whole Church. Therefore, the disposition of truth remains, and Blessed Peter, persevering in the fortitude of the Rock that he accepted, has not relinquished the governance of the Church that he received. [8]" What more needs to be said? The Church has declared here, dogmatically, that the man who succeeds St. Peter's successors receives "the Primacy of Peter over the whole Church". And that's it! Fr. Chekada can respectfully explain away, till he's blue in the face, why the Fathers felt compelled to introduce such a statement. But the fact remains: the statement stands on its own. Who are you to question it? I am no one; I am nothing.
I have read the commentary on this matter by Council Father, His Eminence, Card. Franzelin, the above arguments by Fr. Chekada, and other quotes. But the Church has spoken definitively and infallibly! If anyone will ever even discuss this matter, I imagine they would need an extraordinary permission to do so by a future pope, as it is not the place of just anyone with Holy Orders, much less the laity, to be even discussing these matters, and the only reason we are doing so here is because of the extraordinary times we live in. But the latter is merely a fact; it is not a permission to delve into a theological discussion and opinion without end.