Papal infallibility is a spent bullet. The definition of the Assumption of Our Blessed Mother Mary came out of the "sensus fidelium" that Rome had the good sense to consult in advance. The response of the Catholic world was resoundingly affirmative!
We are all, including the Pope, infallible when at Mass we rise and proclaim the Apostles' Creed. And that's good enough.

Prof. Kung accepts that the Church is indefectible and the Pope is personally infallible. This is the first part of the Dogma defined by Vatican I in 1870. The Council also attached to the same Dogma that indefectibility of the Church extended to the Universal and Ordinary Magisterium of the Church. Kung demands that the extension be revoked. Being part of the Dogma, the Pope or any Council for that matter cannot rescind what is defined. Kung is asking for the impossible, because the Universal and Ordinary magisterium is exercised by the Vatican Curia stacked with Cardinals, Archbishops and Bishops.

Surely the sensus fidelium is that the Pope is not infallible. It is the Church, the Body of Christ, that cannot err.

Prof Kung is right to encourage this debate now, with a new openness in Rome - at least in one person there (and an important one, in the pope).
This doctrine deserves questioning for two reasons. Firstly in the narrow sense, where it is used on a belief like the assumption of Mary. This is an unscriptural belief and is a major barrier to ecumenical dialogue, typifying the inherent problem with the basic concept of papal infallibility.
Secondly, you rightly note the creeping nature of the concept of infallibility. It is extended further, not just by popes like the last two, but also by arrogant Roman congregations, like the doctrinal, sacramental and episcopal departments, who almost assume that it applies to them by extension. Their treatment of even moderate bishops in my country, trying to minister to remote communities, has been appalling.
If this creeping infallibility trend is reversed, there is hope that the church will become more truly an ecumenical world wide body, without the centralising control model that has given us the shame of ex communications of good people, the protected abuse of children and other horrors like the inquisition. We must discover that the institution is the servant, not the master.
Prof Kung has been a great and unheard prophet. It would be wonderful if he was rescued from his time down the well, in his lifetime, and the church finally listened to him and addressed this sad error.