There is no mistake. 'Where the pope is, there is the Church'. An ecuмenical Council, the laws of the Church, the liturgy, are only official when the pope nods his head. If he nods his head, and it goes out to the Church, and there is something found that is harmful, then the man who gave the nod cannot be a true pope. This is a dogmatic fact.
Not dogmatic fact, but dogma. Otherwise, I agree with you. It's the reason that R&R is not acceptable. Now, various R&R nuances along the lines of "I don't know. I'll just give him the benefit of the doubt until the Church declares him deposed." -- those are different. That's more along the lines of a sedeprivationism which grants his material occupancy a certain kind of "status".
Nevertheless, there's a fault in this reasoning process:
If some teaching that cannot be harmful actually brings harm, then the pope cannot be the pope. This is what I refer to
modus tollentis sedevacantism, the other form where you start with the person of the pope
modus ponentis sedevacantism.
Let's say a Pope were to declare a new dogma. According to sedevacantism, if you can't accept the dogma, then you can declare him a non-pope. At that point, no dogma is safe because you can just contest the pope's legitimacy. Let's say I was alive at Vatican I and decided that papal infallibility was an error. What would have stopped me from simply declaring Pius IX a non-pope? In order to maintain the integrity of the Magisterium, we would have to change our minds about infallibility and accept it ... rather than rejecting Pius IX. Why? Because we would have known at the time with the certainty of faith that Pius IX was the pope. And how can we know this with the certainty of faith? Because the Church universally accepted Pius IX. Did anyone dispute the legitimacy of Paul VI before he signed the various problematic docuмents of Vatican II? No. He was peacefully accepted by the entire Church as a pope ... which rendered it a dogmatic fact. Conversely, if we cannot know the legitimacy of a pope with the certainty of faith independently of an
a priori to any of his teachings, then we can never know any teaching or dogma he proclaims with the certainty of faith. Magisterium completely disappears. So straight SVism is no less harmful to the Magisterium than R&R. They both destroy it, one via Magisterium-sifting, the other via Pope-sifting. Both of these constantly subject Church teaching and papal legitimacy to a constant feedback loop of validation based on the acceptance of the pope's teaching ... as determined by private judgment. Pope teaches A. I judge A to be Catholic. Still Pope. Pope teaches B. I judge B to be an error. No longer pope. This is silly.
Whether you admit it or not, this is an incredibly serious problem with sedevacantism.