I had not thought about that. Doubtful episcopacy solves the problem for Ratzinger and sucessors in this case.
All the same, I understand that the Cassiciacuм thesis says that the modernist cardinals are legitimate to some extent, and that they could, in theory, elect a valid Pope.
Right. So, the implications of a doubtful episcopacy are interesting. One must be a cleric at least to undertake certain aspects or functions of the papacy, e.g. start making appointments, etc. Ratzinger certainly was a valid priest (therefore a cleric). But in order to exercise teaching authority, he needed to become a member of the
Ecclesia Docens and one must be a bishop to do so. So he couldn't exercise teaching authority.
So, yes, that's one possible explanation, the other being that by the time he was "elected" there were no legitimate/valid Cardinals left. This would be true for both straight sedevacantists and for Siri Theorists, since for both those groups these papal claimants weren't even "material popes, whereas for the sedeprivationists the Cardinals could still be valid ... in the sense of having the ability to elect.
Until the Church intervenes, we'll never know exactly what happened, and I think too much time is wasted on arguing about these details. I think that what SVs, SPs, and Siri Theorists have in common is that we rejection the notion that the Conciliar Church is the Catholic Church. Rest of it are just attempts to explain, then, how this Conciliar Church was able to somehow eclipse or displace the Catholic Church. And there's a tendency to conflate this final conclusion with one's favorite explanation, and because the conclusion is dogmatically certain, attempting to assert that the explanation is also dogmatically certain. That's a logical fallacy.
I've descried myself as a dogmatic indefectibilist. As far as I'm concerned, any theory that upholds the indefectibility of the Church's Magisterium and Universal Discipline is fair game, and I'm not going to argue too much about it. If one wanted to believe that the real Paul VI was replaced by a big-eared crooked-nosed double, drugged, and held captive in a dungeon, while the imposter who replaced him wreaked all the havoc, well, more power to you. I might not buy it ... but you're NOT rejecting the overall indefectibility of the Church's Magisterium and Universal Discipline, which would be preserved under such a hypothesis.