Rob Banks : This would mean that under sedevacantism, the Church (or at least the clergy) eventually ceases to exist, which would mean that the Church failed.
This is what I tend to believe. I like the sedevacantist group Most Holy Family Monastery, but I can't wrap my head around some of the stuff they believe.
I would argue that on the contrary, sedevacantism is the ONLY logical position in which the Church hasn't failed. In all the other positions, be it SSPX or Novus Ordo conservative, you have zero explanation for the elephant in the room, so you simply ignore it or make mental acrobacies to minimize it. Thus you have EMJ trying to show that Nostra Aetate did not mess up as much as is generally thought. The doctrine of infallibility/indefectibility of the Church does not say that the Magisterium will not mess too much, it says that the Magisterium will never mess up, period.
You have to remember the distinction Church militant/Church expectant/Church triumphant ; the two first are temporary, and only the last is forever. Visiblity of the Church (militant) or perpetuity of the papacy is to last only till the end of the world.
That we are near the end of the world is not a fancy hypothesis or a random guess, there is objective evidence for it. Such as, the legal apostaty (as Leo XIII called it) of all the formerly Catholic States who have all separated from the Church and expelled the religious viewpoint from their constitutions and laws, and now protect abortion and sodomy, etc.
By the way, the MHFM guys are crooks who deceive their clientele about who they really are. The doctrine on their website is mostly OK and Catholic, although it includes some outright heresies such as Feeneyism - the denial of baptism of desire.