I acknowledge that a Pope can defect from the Church through heresy like anyone else, just that it is not the case at this time. One can pick apart all the sedevacantist doctrinal arguments, but it is easier to look at their principles to see why all the different brands of it are wrong in principle.
Sedevacantist claims are purely negative (“the putative Catholic Church is heretical, therefore we by default are the true Church”) resting on certain doctrinal controversies and arguments over whether something is a legitimate theological conclusion or actual heresy (claims that the Catholic Church has fallen into heresy are not new, just ask the Greeks or the non-Chalcedonians), none of which have been definitively condemned by the Church, but which were taught at an ecumenical Council, and IMO can all be defended. They also conflate simply being wrong about something with the heresy that necessarily separates one from the Church.
They have no argument as to why they are the true Church, because they aren’t. Sedevacantism in all its forms is clearly lacking in hierarchical communion, a teaching authority, the primacy in act or potency, and various other elements as well as a perduring continuity–all of which are proper to the true Church. The putative Catholic Church clearly has all these things.
Let’s look at some of these issues in more detail:
(1) Thave no primacy in act or in potency. Since the Roman primacy is a constituent element of the Church and the Church has defined that there is to be a perpetual succession of bishops in the Roman primacy, the Church must retain the primacy either in act (with a living Pope) or in potency (if there is no Pope, the Church retains the power recognize there is no Pope and to appoint a new one). The Church cannot choose to go without a Pope (this was an error of Hus condemned at the Council of Constance). All historical sedevacantists (ie those that thought the Pope fell out of the Church due to heresy, but still maintained the primacy) from Hippolytus’ to Okham’s group, etc., even though they were all wrong, all at least sought to restore the primacy to act. It appears that for sedevacantists, that power to restore the visible head to the Church cannot be found–in fact, they all claim not to have that power at all, from what I can see. Being unable to elect a new head or choosing not to both violate the divine constitution of the Church.
On the other hand, the true Church has not lost this constituent element and does not fail to do this. This is because it also follows that there must be divine assistance involved in this recognition, since if the Church could fail to recognize her headlessness, she could proceed as headless in perpetuity (and even possibly submit to the teaching and governance of someone other than her head), which is impossible given the above and the other divine promises made to the Church regarding her indefectibility and infallibility.