Other than theologians who speak of loss of office due to heresy, another thing that I think of is from scripture. When Christ was betrayed and arrested by judas and the officers of the high priest at the mount of olives, all of the apostles fled and scattered. I think john the apostle even ran off without his clothes to escape. And, I think that may be an indication of what happens when the pope and bishops do not do their job(aka the popes hierarchy is not protected like peter is). The bishops and popes have not not stayed awake to watch with Christ for even 1 hour. Christ foretold of peters thrice betrayal, and it of course happened. But, all of the apostles scattered. So, where was the church at that time? John the apostle tradition says was with our lady as seen by them together at the foot of the cross. Where two or more are gathered together, there Christ is in their midst. There is the church. And, Christ promised Peter his faith would not fail. So, despite his thrice denial, Christ was with Peter. Two or more, and there is the church. But, the church is one. So, all of them were in a real sense, together as one church. That is what I see of the balancing act or tight rope +Lefebvre tried to walk. There is the church. However, there is not enough in the scriptures during that time to suppose that a heretic or denier who does not follow Christ during a time of crucifixion for the church retain their legitimacy/place in the church. It was not until after the crucifixion that more apostles went fishing with peter to suppose that the other apostles were legitimate at that time. These are just my thoughts. Because, I really think that if there can ever be a deal made between sspx and doubtism/vacantism, it hinges for one thing on whether novus ordo bishops are legitimate. The sspx can never give up believing the pope is valid and legitimate, so vacantists will have to agree to that. But, are the local bishops legitimate? The whole sspx marriage issue revolves around this question. And, I have my doubts.