Sedevacantism contradicts itself. Sedes over the years have disagreed among themselves on who was the last valid pope. Since there must have been a last valid pope, the first imposter pope (presumably appointed by him) would at least have been a valid bishop; so he at least would have valid episcopal authority.
How can you measure when a pope is an imposter? If you say “doctrinal infidelity”, how do you measure that? You might say Pope X disagrees with these prior popes…but how do you know the prior popes were not imposters? Where did you get your template?
Obviously there has been development of doctrine over time. Sedevacantists accept the Trinity, the innovation of a New Testament canon, and a whole slew of other post - St. Peter developments, as fitting into the “core” Catholic belief. But if you accept developments as possible, how can you label Pope Y an imposter if he supported development? Or rather, why don’t you label prior popes (who you use as your comparison template) imposters if they supported innovations like the New Testament?
In order to label some pope an imposter, sedes assume there are criteria for validity, that these men do not meet. Well where did those criteria come from? If you say “rigged election”, well, how did you get the idea there should be an “election” at all, let alone a fair one for a valid pope?
The problem is sedes never show how they justify the validity of past doctrinal developments, or validity of past popes that they claim later popes are inconsistent with. Sedevacantism is essentially a form of Protestantism, in that they point to an imaginary point in the past, and say “obviously” that is all true, but changes after that are wrong. I don’t ask why they say modern stuff is wrong, I ask why they put earlier developments and changes on such a concrete foundation, to use it as a template.
A sedevacantist does not really think there is no current pope. The sede in effect puts himself into the papal chair.
