Historically, sedeprivationists have held that the Pope who is not formally Pope may still be able to effect some acts validly, so that thereby the Church can be continued. These include appointing Cardinals, or incardinating Roman clergy, and at least some have argued that they would extend to appointing Ordinaries. Some of them denounced simple sedevacantism as heretical because it provided no mechanism for this, also because most sedevacantists held to the idea that the whole hierarchy had lost their offices and all episcopal sees were formally vacant.
More recently, some sedevacantists, although rejecting sedeprivationism, still seeing the need for the same appointments to be valid, believe that although the Pope is not the Pope in any sense, still these acts of his could be validated by supplied jurisdiction. Almost all can see that the Church cannot be continued otherwise, when all existing Ordinaries die, therefore such a theory is proposed.
But both of these seem impossible to reconcile with, among other things, Pope Paul IV's Bull Cum Ex Apostolatus, stipulating that
each and all of their words, deeds, actions and enactments, howsoever made, and anything whatsoever to which these may give rise, shall be without force and shall grant no stability whatsoever nor any right to anyone
Therefore, if the Pope is not the Pope, then those appointed by him have neither right nor stability, meaning bishops have neither office nor jurisdiction and Roman clerics cannot be incardinated by his acts, for they are null and void. The bishops and clerics, then, and the validity of their appointments stand or fall with the Pope's own claim. If he is really the Pope, then his enactments are valid and give rise to real bishops and clerics with a mission, ordinary jurisdiction and lawful incardination. If he is not, all his acts are as null and void as his Papacy itself.
The Church is indefectible ... in her monarchical constitution (principle 4), comprising governing power i.e., jurisdiction, hence Vatican I’s profession that Peter will have perpetual successors ... is indefectibility preserved if there is no pope since 1962 or if there is no one with ordinary jurisdiction whom the sedevacantists can point out as such?
There are only about 15 ordinaries left in the world appointed by Pius XII, if we include bishops appointed by John XXIII, their number increases by about 10, almost all of these near death and emeritus. When these die, the formal Apostolic succession has ceased, the ecclesia docens
is no more, ordinary jurisdiction has not been passed on, and the promise of Christ has failed. Either that, or 50+ year sedevacantism is false.