Nobody has the "DUTY" which you descibe, nor the responsibility of judging popes. As Pax vobis says: emotional, illogical, wrong...
You make your decisions for yourself and for no other.
The people in the 14th century had to make a judgment whether Urban or Clement was the true Pope, and after they made that judgment, they followed the Pope of their choice entirely
in matters of faith and morals. The sedevacantists are not doing much differently; they're making a judgment on whether Francis is a true Pope, and in their case not following him at all because they don't believe he is true based on prior magisterial statements. The sedevacantist bishops and priests then tell their flocks to stay away. This is normal
as we see from this book in 1906.
Also keep in mind that those priests following Pope Clement were otherwise entirely
Catholic, and the validity of their orders and Sacraments were not in question. Likewise for the priests and faithful following Pope Urban. The ONLY thing that changed during the Western Schism was that these priests and laypeople on each side were believed to suddenly be following a false Pope. Notice that this alone
was enough for those who followed Pope Urban to stay away from the Clementine Masses (and vice versa), even though those masses were VALID. Why? Because by following anyone other than a true Pope it is considered schismatic by definition, and as we know, the Church has always stated it is the duty of every Catholic to avoid schismatics.
The bottom line is, the sedevacantists are reacting the same way as the Catholics did during the Western Schism and should not be faulted for doing so.