In other words, my understanding is that the objection is as follows:
Major: It is impossible that all the episcopal sees of the world be vacant simultaneously.
Minor: But sedevacantism says that all the episcopal sees are vacant simultaneously.
Conclusion: Therefore sedevacantism states something impossible, and is therefore false.
Yes, that's right. That's the syllogism.
We agree I think that Straight or Simple Sedevacantism (hereafter SS) leads to EVism.
It remains for us to examine if the major is true. If it were true, the conclusion would follow.
There are perhaps 3 dogmatic statements that could be provided, before we look at texts of theologians.
First, the statement of Vatican I, that there will always be Shepherds and Teachers in the Church, which seems to plainly refer to Bishops with Teaching Office and Ordinary Jurisdiction, "as he sent apostles, whom he chose out of the world [39], even as he had been sent by the Father [40], in like manner it was his will that in his Church there should be shepherds and teachers until the end of time."
https://www.ewtn.com/catholicism/library/first-vatican-council-1505Shepherds and Teachers,
pastores et doctores in Latin, seems to describe Bishops who have Power of ruling i.e. jurisdiction, and of teaching, i.e. magisterial authority, doesn't it? Well, such would only come from appointment to an episcopal office/see/diocese.
Second, it follows from the Apostolicity of the Church. For the Church cannot cease to be Apostolic, and Apostolicity requires Bishops who have succeeded to episcopal sees, i.e. it requires ordinary jurisdiction not only valid orders. Another statement in Vatican I itself speaks of "that ordinary and immediate power of episcopal jurisdiction, by which bishops, who have succeeded to the place of the apostles by appointment of the Holy Spirit" from which the same conclusion would follow. So, what is requisite for apostolic succession? That Bishops have ordinary power of episcopal jurisdiction, and be appointed to an office/see to which it is attached.
There are various theological descriptions of this also in the manuals and one other dogmatic statement that we can look into later on.
Your thoughts on this, Yeti?
God bless.