I owe this quotation (taken from a book which Rahner, one of the great Conciliar liberals, wrote in 1965) to the magnificent Phoenix from the Ashes by Mr Henry Sire (Angelico Press).
"Imagine that the pope, as supreme pastor of the Church, issued a decree today requiring all the uniate churches of the Near East to give up their Oriental liturgy and adopt the Latin rite ... the pope would not exceed the competence of his jurisdictional primacy by such a decree, and the decree would be legally valid. But we can also pose an entirely different question. Would it be morally licit for the pope to issue such a decree? Any reasonable man and any true Christian would have to answer 'no'. Any confessor of the pope would have to tell him that in the concrete situation of the Church today such a decree, despite its legal validity, would be subjectively and objectively an extremely grave moral offence against charity, against the unity of the Church rightly understood (which does not demand uniformity), ... a mortal sin from which the pope could be absolved only if he revoked the decree."
Rahner was right. An action can be "legally valid" and, at the same time, be totally wrong. There is a Something which can trump mere "legal validity", and that Something is Holy Tradition.
I would not, myself, play around as Rahner does with speculations about what papal confessors might or might not say in the confessional. I would certainly not join Rahner in his exalted view of the "competence of [the pope's] jurisdictional primacy" ... a view which seems to me to be a big part of our current problem.
I would simply say that such a pope, and such a decree, would lack all AUCTORITAS, because they flout Holy Tradition.
Such an action would be, as Sire observes, "not merely a sin, ... but an act of schism against the tradition of the Church, and one for which [a pope] has neither moral nor legal competence."
Rahner's observations, of course, hold true also of the Roman Rite, which has just as much auctoritas as the Oriental Rites. It is, in fact, older than they are.
PF's evident policy of destroying the Authentic Roman Rite, and his recent purported legislative action, lack auctoritas and place no burden upon anybody's conscience. I am confident that, one day, it will be declared that they were null.