At most, they are material schismatics. But they would never want to be separated from the Church Christ founded.
Eastern Orthodox and Protestants do not want it either. Ask any Orthodox or Protestant. They also have good intention. They don't think themselves as being separated from the Church Christ founded.
In Our Catholic Church, the visible foundation of the Supreme Pontiff is the glue that has hold us together for more than 2000 years. When you cross the line and cast off the visible monarchy of the Church, you open a pandora box and you eat your own tail. I can sympathize with the sedevacantist position but let us not support the error.