The guarantee is that the Church as a whole is infallible. She may be tempted but she won't fall as a whole. It is the prayer of Christ specifically for St. Peter that he may not fall and once converted, confirm his brethren (Lk 22:32). It is a matter of faith, of course.
You also exaggerate the negative role of the past popes. None of them had an evil intention; most abuse occurred by the faithless clergy against the backdrop of rising neo-paganism. Some frivolity for sure was tolerated, and it is very regrettable, but no pope "took the Church in the wrong direction", at least not permanently -- provided, indeed, that the new movement toward liturgical orthodoxy is sustained. At this moment it looks very good. I firmly believe that the worst is behind us.
“None of them had an evil intention”
You are kidding, right?
What about the Borgia Popes, or the Medici Popes?
These were about the most unrepentant reprobates to ever sit on the Throne of Peter!
We are not worried about the Church failing, we are worried about being taken on a journey a la another Vatican II that may take a centuries to heal, if ever. I would not want to put my Church in the hands of one man. Some guarantees must precede any reunion.
You also exaggerate the negative role of the past popes. None of them had an evil intention; most abuse occurred by the faithless clergy against the backdrop of rising neo-paganism
No, I am sure they didn't but I don't believe the outcome of the Vatican II was an undesired accident, or else they were deceived. The Vatican II obviously has "holes" that allows a wide range of interpretation. Was that deliberate or accidental?
Bishop Mahoney in LA, for example is still a bishop there because he is not doing anything that is uncanonical. Otherwise I am sure he could be removed. This means the "holes" are big enough to accommodate practices seen in his church, which is just about "anything goes." Right?
Obviously the popes were not interested in annulling the Vatican II. Somehow, everyone claims the Vatican II was (a) necessary and (b) good for the Church and (c) no one wants to to go back to the old "bad" ways, but judging from the reform of the liturgical reform it is beginning to look more and more like the old ways.