This argument isn't about infallibility in the strict sense, as defined by Vatican II. It's about the indefectibility of the Church.
R&R (as represented by SeanJohnson and a few others here on CI ... NOT Archbishop Lefebvre) basically blaspheme (and would have been burned at the stake on orders of a St. Robert Bellarmine), and they destroy the Catholic Church ... all to rescue Bergoglio.
If the Church founded by Our Lord Jesus is capable of ...
1) holding an Ecuмenical Council that has led the entire Church astray into Modernism
2) replaced the Catholic Mass with pile of rubbish with dubious validity and also blasphemous (replacing the Offertory with a тαℓмυdic table prayer)
3) taught garbage to the Church for >60 years now (where most of their Encyclicals aren't fit to be used for toilet paper)
4) canonized some of the greatest evil-doers in the history of the Church
5) has led sols to hell for >60 years,
then there's no use for a Church, then the Prots were right (while being wrong on the details), and the Old Catholic were right, and the Eastern Orthodox are right, and the promises of Our Lord meant nothing, and the Catholic Church would just be a pile of rubbish overall. Absit.
In short, we behold here in the Conciliar Church a brand new religion.
Never mind blaspheming the Holy Catholic Church, just to save Bergoglio.