Alexander VI actually did his job
Pope Alexander VI was also accused of heresy.
For I bear witness in the name of God that this Alexander VI is in no way Pope and cannot be. For quite apart from the execrable crime of simony, by which he got possession of the [papal] tiara through a sacrilegious bargaining, and by which every day he puts up to auction and knocks down to the highest bidder ecclesiastical benefices, and quite apart from his other vices - well-known to all - which I will pass over in silence, this I declare in the first place and affirm it with all certitude, that the man is not a Christian, he does not even believe any longer that there is a God; he goes beyond the final limits of infidelity and impiety
To understand why the accusation was false is to understand why modern sedevacantism is false. Cardinal Billot treats this example in detail.
But of course you won't tell us what the argument was.
God cannot…permit that the whole Church accept as Pontiff him who is not so truly and legitimately. Therefore, from the moment in which the Pope is accepted by the Church and united to her as the head to the body, it is no longer permitted to raise doubts about a possible vice of election or a possible lack of any condition whatsoever necessary for legitimacy. For the aforementioned adhesion of the Church heals in the root all fault in the election and proves infallibly the existence of all the required conditions.
Can I assume that this is what you are referring to?
But does the whole Church accept Francis as Pontiff? I count over 40 Catholic bishops and hundreds of Catholic priests who do not. I also see that there is much doubt in the minds of many who still "recognize" Francis as the pope. I have heard many times from them that if a future pope declares the V2 popes to be antipopes they will accept the decision. So one can hardly claim that the whole Church accepts Francis as the Pontiff. In fact, an argument can be made that what the entire Conciliar Sect thinks about Francis' claim to the papacy is completely irrelevent because they are not even members of the Church. So really the disagreement is only between those Catholics who adhere to the Recognize and Resist position and those Catholics who adhere to the sedevacantist position. If you go by that standard, the R&R group has a majority but certainly it cannot reasonably be claimed that the whole Church accepts Francis.
That being said, I agree with Cardinal Billot's assessment of Pope Alexander VI. But if Savonarola had been successful in convening a council and proving his accusations and causing the council to condemn Alexander then it would have been a different story altogether.
To see or to know something is not to judge. -St. Francis de Sales.
Through tacit resignation, accepted by the law itself, all offices become vacant ipso facto and without any declaration if a cleric: ...n.4. Has publicly forsaken the Catholic Faith.
[8] But though we, or an angel from heaven, preach a gospel to you besides that which we have preached to you, let him be anathema. [9] As we said before, so now I say again: If any one preach to you a gospel, besides that which you have received, let him be anathema.
There is no Catholic God.