Imagine you are a r&r, and then you meet a Protestant who believes in faith alone.
You prove to him how the Catholic Church is the real and only Church and how he's wrong and how he need to become a Catholic if he wants to be saved.
Just when he is about to convert, he finds out about the 1999 Joint Declaration with the Lutherans which what you, as an r&r, deem to be the Church and your authorities, accepted.
Then he will say "Wait a second! I can still remain a Lutheran according to your Church! This is what ecuмenism is all about! I can still believe in sola fides according to this docuмent! And moreover, we will jointly commemorate the 500th anniversary of Luther's tacking of his theses!".
He will also point you out to the fact that what you deem to be the Church, rejects proselytizing and making converts. He will show you how Ratzinger, as Benedict 16, explicitly told Protestants in 2005 that they don't have to convert. He will point you out to the FACT that what you think is the Church blatantly rejects the salvation dogma and teaches you don't even have to be Catholic, or accept the papacy etc.
As a r&r, your position boils down to telling him: "the last 6 Popes are wrong, and the entire Hierarchy is wrong too. We are right and the Church is wrong".
He will logically tell you: "Oh and who made you Pope? How are you not setting yourself up as the Magisterium? How are you any different from any Protestant since you decide for yourself what you will believe and you reject and defy authority? If they are your Popes how can you challenge them? Moreover your own Church regards you as schismatic and as Pelagians".
Any non-Catholic will tell you: "No thanks! YOUR POPES AND YOUR AUTHORITIES SAY OTHERWISE".
You will be left utterly defeated, embarrassed, and utterly incapable of making a single convert.
If you hold to r&r that is.
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This is just one of countless examples i could give of the utter absurdity of recognizing these men as Popes and real authorities, because you cannot even logically defend the Faith or make a convert.
It will always boil down to "the Pope is wrong, we are right" and that is completely un-Catholic.
And of course the issue isn't about a pope's private opinion on things, so don't even go there.
We are talking about the fact that the r&r people reject what they deem to be the Pope's and the Church's Ordinary and Universal Magisterium, which is infallible.
The sedevacantist avoids all these absurdities by proclaiming the truth: all these guys are apostate non-Catholic heretics and they do not represent the truth, and it is precisely because of those things which you want to justify not converting that they are not Catholics. We reject them completely and they have no authority whatsoever.
The sedevacantist is the only one who can fully defend the Faith right now without being embarrassed.