Papal infallibility seems at first glance to render the Magisterial deposit of dogma quite vulnerable to corruption by heretical innovation on the part of errant popes. But actually it works the other way: because popes are infallible in their ex cathedra pronouncements on matters of faith or morals, *no pope can coherently contradict such statements by previous popes.*
Whether or not a new pronouncement does in fact contradict dogma is of course a matter of interpretation, and can be decided only by means of Magisterial discourse – not just among bishops and theologians, but also among the laity, priests and deacons; all of whom together embody and carry forth the sensus fidei.
The Magisterium is the thought and memory of a living organism. Organisms coinhere, and so subsist – and so, live – in virtue of their massive, dense intracommunication. By that continuous discourse is the Magisterium carried forward from each generation to the next. The Magisterium, then, is a conversation. The sedevacantists are participants therein.
The key thing to bear in mind is that true *participation* of the Magisterial conversation requires mutual loyalty among interlocutors, especially to those who have come before [Exodus 20:12]. He cannot participate the Magisterium who construes himself as having rejected it, and left it.