Are you trying to convince the sedevacantist or yourself with this topic?
Just curious!
Convince others, of course.
Here is a parable, which kind of explains why I get upset when people attack the OLD SSPX, on this Resistance-friendly forum that regularly bashes the neo-SSPX:
John's mother, at the age of 60, decided to divorce her husband of 40 years and run off with a younger man, who also happens to be much more adventurous than her old husband ever was. Their first marriage was in the Catholic church, and certainly valid.
John was complaining about this to a few friends and coworkers of his, how this causes all kinds of problems for him, especially now that he has children of his own.
One of his co-workers pipes up, "yeah, your mother always was a whore, wasn't she!".
John punches the co-worker in the face for talking about his mother that way.
The co-worker, in disbelief, and wiping a bit of blood off his lips with his right sleeve, bellows out,
"Why did you hit me for? You're complaining about your mother. I'm on YOUR side, against her, right?"
John explains: "No, you're not. I'm complaining about a fall that took place when she was 60 years old. She married my father with all good intentions 40 years ago. She was faithful to him all those years. Until she went crazy recently, I had nothing to reproach her about. You suggest that she was always a whore, and that simply isn't true!"