The problem wth believing this (and still desiring to call oneself Catholic) is that in order to elect a Pope, a college of Cardinals is required. And Cardinals are appointed by the Pope. These are simply facts every Catholic knows so don't ask me to "prove" them, because if you do you will expose yourself as not truly Catholic.
So the problem is, that since it takes a college of Cardinals to elect a Pope, and all the Cardinals since Pius XIIth was elected are dead, there is no (truly Catholic) way to elect another "true Pope".
So congratulations you've just made yourself a Protestsnt.
ROFL
You posited two concepts.
One that it is impossible to regain a pope when all Cardinals have apostacised.
Two that I am a protestant.
First of all it is not beyond God to provide what is needed to restore a pontif to the world in a situation that was in many ways unforeseen by man. He has done it before.
Secondly, I would suggest that those who follow the Novus Ordo have made themselves Protestants in so doing. So called Paul VI invited Protestants to the so-called Vatican II council in order to destroy the Mass and to Protestantize it. Only a true Catholic opposes the heresy of this anti-pope.
Sedavacantists have merely sought a more realistic ground for remaining true to the One Holy Catholic Apostolic Church. We attend the same Mass celebrated for over a thousand years, and offered by priests ordained in Apastolic succession.