Let me explain something to you. You can't connect the dots or draw all the conclusions that way. You just can't do it, otherwise there is no discussion between the groups. Both sides condemn the other side to hell, and that's the end of it. It's not that simple!
Your little syllogism sounds pretty airtight. But watch this --
Christ promised "The gates of hell shall not prevail against it"
But sedevacantists claim we haven't had a Pope in 62+ years.
Therefore sedevacantists deny Christ's promise.
The Church teaches that the Pope is the highest office, which has no earthly judge.
But Sedevacantists throw out whatever popes they judge to be in heresy.
Therefore they are, de facto, higher than the Pope(s), since they can depose him (them).
You see, it's not that simple. You will find that sedevacantists have *at least as many* unanswered questions (weaknesses, Achilles' Heels) as other positions. None of the positions are simple, airtight, or perfect. If any of them solved the Crisis and answered all the difficult questions satisfactorily, it would follow that at least all men of good will with a decent IQ would have adopted that position! But that is not what we observe in the real world. Ergo.
And my favorite argument:
The solution to the Crisis in the Church will solve the Crisis, bringing the Church back to normalcy.
But Sedevacantism as such hasn't done ANY good, over and above the good of the Traditional Movement at large, in 6 decades.
Therefore Sedevacantism is not the solution to the Crisis in the Church.
Sedevacantism is just The Traditional Movement with a dart board bearing the Pope's face, which you throw darts at. In other words, useless. I don't need that add-on. I'll take Traditional Movement Classic, thanks. That's where 100% of the good fruit is, and has always been, up to and including the present day.
Even the good fruits at various "sedevacantist" chapels today is because they are Traditional Catholic (Traditional Movement), not because they are specifically sedevacantist. That's my point.