There's been more than one successor to Peter, Mark. The current successor dished one of his predecessors in regards to the Syllabus of Errors.
What say you?
I say that it is up to the Magisterium. That is whom the Lord left in charge. There have been many individual bishops over the years who have rebelled and formed their own theological groups.
Now, I am not accusing SSPX of pulling an Arius or Montanus or Nestorius, but I would view them as wandering in that direction. Probably a more accurate parallel would be the bishops who rebelled after Nicea in 325. Some of them were very well meaning. However, they were wrong.