I think we need to be nuanced here, which you may be by using “tradition” with a lower-case t, but I still think a clarification is in order. The Popes do not get to decide what is Sacred Tradition anymore than they get to decide what is Sacred Scripture. The office of the Papacy is to defend, interpret, and transmit Sacred Tradition, not change it. By its very definition, Tradition cannot be changed. What Our Lord taught the Apostles is not subject to change.
Hi

, just in case you are unaware, this is very much the ‘party line’ of the SSPX and is their repeated defense to justify and hold their “untenable” position. Here are some extracts from the Letter of Pope Paul VI to Archbishop Lefebvre in 1976, that make the point more clearly than we can:
“What is indeed at issue is the question - which must truly be called fundamental - of your clearly proclaimed refusal to recognize in its whole, the authority of the Second Vatican Council and that of the Pope. This refusal is accompanied by an action that is oriented towards propagating and organizing what must indeed, unfortunately, be called a rebellion. This is the essential issue, and it is truly un-tenable.”
“In regard to the Pope, every Catholic admits that the words of Jesus to Peter determine also the charge of Peter’s legitimate successors: “. . . whatever you bind on earth will be bound in heaven” (Mt. 16:19);…“The pastors of every rank and of every rite and the faithful, each separately and all together, are bound by the duty of hierarchical subordination and of true obedience, not only in questions of faith and morals, but also in those that touch upon the discipline and government of the church throughout the entire world. Thus, by preserving the unity of communion and of profession of faith with the Roman pontiff, the church is a single flock under one pastor.** Such is the doctrine of Catholic truth, from which no one can separate himself without danger for his faith and his salvation”** (Dogmatic Constitution
Pastor Aeternus, Ch. 3, DZ 3060).”
“Each bishop is indeed an authentic teacher for preaching to the people entrusted to him that faith which must guide their thoughts and conduct and dispel the errors that menace the flock. But, by their nature,** “the charges of teaching and governing . . . cannot be exercised except in hierarchical communion with the head of the college and with its members”
(Constitution Lumen Gentium, 21; cf. also 25).* A fortiori*, a single bishop without a canonical mission does not have
in actu expedite ad agendum, the faculty of deciding in general what the rule of faith is or of determining what tradition is.”
“In practice you are claiming that you alone are the judge of what tradition embraces.”**
“You say that you are subject to the church and faithful to tradition by the sole fact that you obey certain norms of the past that were decreed by the predecessor of him to whom God has today conferred the powers given to Peter. That is to say, on this point also,
the concept of “tradition” that you invoke is distorted.”
“Tradition is not a rigid and dead notion, a fact of a certain static sort which at a given moment of history blocks the life of this active organism which is the church, that is, the mystical body of Christ.** It is up to the Pope and to councils to exercise judgment in order to discern in the traditions of the church that which cannot be renounced without infidelity to the Lord and to the Holy Spirit - the deposit of faith - and that which, on the contrary, can and must be adapted to facilitate the prayer and the mission of the church throughout a variety of times and places, in order better to translate the divine message into the language of today and better to communicate it, without an unwarranted surrender of principles.”**
**“Hence tradition is inseparable from the living magisterium of the church, just as it is inseparable from sacred scripture. “Sacred tradition, sacred scripture and the magisterium of the church . . . are so linked and joined together that one of these realities cannot exist without the others, and that all of them together, each in its own way, effectively contribute under the action of the Holy Spirit to the salvation of souls” **(Constitution
Dei Verbum, 10).”
“With the special assistance of the Holy Spirit, the popes and the ecumenical councils have acted in this common way**…Nothing that was decreed in this Council, or in the reforms that we enacted in order to put the Council into effect, is opposed to what the 2,000-year-old tradition of the church considers as fundamental and immutable. We are the guarantor of this, not in virtue of Our personal qualities but in virtue of the charge which the Lord has conferred upon Us as legitimate successor of Peter**,…”
“You say moreover that you do not always see how to reconcile certain texts of the Council, or certain dispositions which We have enacted in order to put the Council into practice, with the wholesome tradition of the church…It is not the place, in this letter, to deal with each of these problems… Absolutely secure counsellors, theologians and spiritual directors would be able to help you even more, with God’s enlightenment, and We are ready to facilitate this fraternal assistance for you”
“But how can an interior personal difficulty - a spiritual drama which We respect - permit you to set yourself up publicly as a judge of what has been legitimately adopted, practically with unanimity, and knowingly to lead a portion of the faithful into your refusal?”