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2. Even if you want to put sedevacantism on the "not erroneous and/or sinful" bookshelf, even then you have a problem here. Apart from the "doctrine" of sedevacantism, we also have their "attitude" to deal with. I'm sure you know what I'm talking about. The above excerpt from the Archbishop illustrates this nicely (thanks Sean).

Don’t expect an answer from the Grand Poobah.  You are too lowly for him to waste any time on you.  He is too busy being the top theologian for the resistance.  And since you are obviously a dogmatic sede (we believe that theoretically it is possible that non-dogmatic sedes exist but we have never run across one in the wild), we shan’t waste any of our valuable time trying to convert you to the true religion headed up by our esteemed Pope Francis, whose portrait adorns our chapels and who we are subject to as evidenced by our naming him in the canon of our Masses even if it is true that we systematically reject his authority over us in every imaginable way.  If you even think there might be something wrong with our position, you are not Catholic, unless, of course, you attend a Novus Ordo Mass (which is harmful for you), in which case, you are indubitably Catholic. Capish?