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At least dogmatic sedevacantism is logical. The theological points they bring up are potentially valid. They apply the exact same logic everywhere and to everyone.

Your doctrine is ad hoc , artificial , entirely man-made. You have to invent that the Thuc line is doubful to pretend that only the line from Bp Lefebvre is valid and you have to pretend that the SSPX is fundamentally differrent from you despite that they do the same una cuм mass.

Yes, dogmatic SVism (though don't accept Kimmage's attempt to classify all SVism as "dogmatic" ... we're not -- in fact, he's a self-admitted dogmatic Willaimsonist), the issue with SVism is, in a way that it's TOO logical, ie. too simplistic and oversimplified, ignoring the "distinctions" that others who don't accept their views might make, and completely ignoring the existence of a MINOR at all, where they equate any rejection of their Conclusion with the rejection of the Minor.  Now, increasingly greater numbers of Traditional Catholics do precisely that, but it's not necessarlly the case.  It's like what I say about BoD, where you could in theory hold it in a way that's not heretical, BUT ... 95%+ of Trads "explain" BoD in such a way as to be effectivelyi holding to the tenets of Pelagianism and a denial of Trent's teaching regarding the necessity of the Sacraments for salvation.

from the SV standpoint ...

MAJOR:  Pope can't teach error to the Church.
MINOR:  Montini [or insert name of any given Conciliar papal claimant] taught error to the Church.
CONCLUSION:  Therefore, Montini canot be the pope.

I mean ... seems pretty darn "logical", right?  So logical that it's "airtight".  But that's only an illusion of the FORM of this syllogims.  Because it's formally so incredibly solid, it's easy to snooker people into accepting it as if it were dogmatically true, or absolutely true, almost as if with the above you were arguing that 2 + 2 = 4.

But ... "DISTINGUO !" ...  D&R (as we should strive to dump the term R&R) DISTINGUISH the Minor.  While the SVs assert it simpliciter, as if it were an absolute truth, D&R would RIGHTLY add the following distinction ...

MAJOR:  Pope can't teach error to the Church. -> MAJOR:  Pope can't teach error to the Church when his teaching meets the notes of infallibility.

That's why the SV mind has spiralled down into what I refer to as something of an intellectual "vapor lock", since they feel the need to reject this distinction forcefully, and therfore asset the opposite, that the Pope is always inerrant in teaching the Church (rendering the notes of VI entirely moot).

Now, let's look at the Minor,, "Monitni [aut al.] taught error to the Church."

So, all Trads would agree on this, but ... we cannot know this with dogmatic certainty.  We have not a few intellectual Catholics who have applied a "hermeneutic of continuity" to V2, where they claim it it's necessarily error but CAN in fact be interpreted in a Traditional sense.  Therefore, since the MINOR is not dogmatically certain, in conjunction with various degrees of distinguishing the Minor, since the Conclusion can be no more certain than the weakest premise of the arugment (peiorem partem sequitur conclusion), the conclusion also cannot be dogmatically certain.  And hereby doth the bell toll for dogmatic sedevacantism.

We can move toward a more balanced view which I have referred to as dogmatic INDEFECTIBILISM.

Here's another hypothetical debate.

SV:  Pope cannot teach error to the Church, so Montini could not have been the Pope.
Non-SV:  Montini was a legitimate Pope, Paul VI.
SV:  So you deny that the Pope cannot teach error?
Non-SV:  No.
SV:  Then how can he be pope.

Non-SV:  Montini was drugged, tied up in a dungeon, replaced with a big-eared crooked-nosed double with a different voice print.

SV: ????

So, here's a case where ... [at risky of inducing an ear worm with that Billy Joel song] ... "You may be right.  I may be crazy.  But it might not be a heretic you're looking at."

Or, someone might aruge that the V2 papal claimants were blackmailed (on account of say, sodomy) and were not acting freely, and that therefore their acts were null and void.

And if you listen to a talk given by +Lefebvre, he explicitly affirm the SV Major, saying that "In this I agree with you, that this is not possibler that the Pope, protected by the Holy Ghost, could destroy the Church to this degree."  But he goes on to say, "Could he have been drugged?  Blackmailed? etc. etc.",  While he dismisses those as extremely unlikely, I think that those questions left just enough room for "doubt" that he also had the same degree of doubt about the conclusion, namely, the non-papacy of the Conciliar papal claimants.