Pope Francis gave an address on the liturgical reform of Pope Paul VI
today, speaking to participants of the 68th Italian National Liturgical Week. In it, Francis declares: "After this magisterium, after this long journey, we can affirm with certainty and with magisterial authority that the liturgical reform is irreversible."
Francis' remarks ironically read like a Quo Primum for the Novus Ordo. Pope St. Pius V's Quo Primum (1570), which has never been revoked or abolished by any pope, decreed that the Traditional Latin Mass, which the saintly pontiff promulgated in accord with the directives of the Council of Trent, would be "valid henceforth, now, and forever" and "cannot be revoked or modified, but remain always valid and retain its full force." Furthermore, St. Pius V warned that if anyone, including any future pope (by implication), would alter his missal, they would "incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul".
Pope Benedict XVI, in Summorum Pontificum,
reiterated that the Traditional Latin Mass "was never juridically abrogated and, consequently, in principle, was always permitted."
Benedict continued: "What earlier generations held as sacred, remains sacred and great for us too, and it cannot be all of a sudden entirely forbidden or even considered harmful.
For Francis, however, not the Traditional Latin Mass, but the reforms that deformed it are what are truly "irreversible."
Below are relevant translated excerpts of Pope Francis' address:
There are two directly linked events, the Council and the Reform, which did not flourish suddenly but after long preparation. What was called the liturgical movement testifies to it, and the answers given by the Supreme Pontiffs to the hardships perceived in ecclesial prayer; when a need is sensed, even if the solution is not immediate, there is a need for it to be put in motion.
The Second Vatican Council made later to mature, as good fruit from the tree of the Church, the Constitution on the Sacred Liturgy Sacrosanctum Concilium (SC), whose lines of general reform respond to real needs and to the concrete hope of a renewal; it desired a living liturgy for a Church completely vivified by the mysteries celebrated.
The direction traced by the Council took form according to the principle of respect for sound tradition and legitimate progress (cf. SC 23)  in the liturgical books promulgated by Blessed Paul VI , well received by the same bishops who were present at the Council, and for almost 50 years universally used in the Roman Rite. The practical application, led by the Episcopal Conferences of their respective countries, is still ongoing, since it is not enough to reform the liturgical books in order to renew the mentality. The books reformed according to the decrees of Vatican II have introduced a process that demands time, faithful reception, practical obedience, wise celebratory implementation, first of all, on the part of ordained ministers, but also of other ministers, cantors, and of all those who participate in the liturgy. In truth, we know, the liturgical education of pastors and the faithful is a challenge to face ever anew. The same Paul VI , a year before his death, told the cardinals gathered in Consistory: "The time has now come definitely to leave aside divisive ferments, which are equally pernicious on both sides, and to apply fully, in accordance with the correct criteria that inspired it, the reform approved by Us in application of the wishes of the Council." 
And today, there is still work to do in this direction, in particular by rediscovering the reasons for the decisions made with the liturgical reform, by overcoming unfounded and superficial readings, partial receptions, and practices that disfigure it. It is not a matter of rethinking the reform by reviewing its choices, but of knowing better the underlying reasons, even through historical documentation, of internalizing its inspirational principles and of observing the discipline that governs it. After this magisterium, after this long journey, we can affirm with certainty and with magisterial authority that the liturgical reform is irreversible.
The task of promoting and guarding the liturgy is entrusted by right to the Apostolic See and to the diocesan bishops whose responsibility and authority I rely on very much at the present moment; National and diocesan liturgical pastoral bodies, training institutes and seminaries are also involved. [...]
 The reform of the rites and the liturgical books was undertaken immediately after the promulgation of the Constitution Sacrosanctum Concilium and was brought to an effective conclusion in a few years thanks to the considerable and selfless work of a large number of experts and bishops from all parts of the world (Cf. Sacrosanctum Concilium, 25). This work was undertaken in accordance with the conciliar principles of fidelity to tradition and openness to legitimate development (cf. ibid . , 23); and so it is possible to say that the reform of the Liturgy is strictly traditional and in accordance with the ancient usage of the holy Fathers (cf. ibid. , 50; Institutio generalis Missalis Romani, Prooemium, 6). ( John Paul II , Lett. Ap. Vicesimus quintus annus, 4).
 "The popes attention is drawn today once more to a particular point of the Churchs life: the indisputably beneficial fruits of the liturgical reform. Since the promulgation of the conciliar Constitution Sacrosanctum Concilium great progress has taken place, progress that responds to the premises laid down by the liturgical movement of the last part of the nineteenth century. It has fulfilled that movements deep aspirations for which so many churchmen and scholars have worked and prayed. The new Rite of the Mass, promulgated by Us after long and painstaking preparation by the competent bodies, and into which there have been introduced-side by side with the Roman Canon, which remains substantially unchanged, other Eucharistic Prayers, has borne blessed fruits. These include a greater participation in the liturgical action, a more lively awareness of the sacred action, a greater and wider knowledge of the inexhaustible treasures of Sacred Scripture and an increase of a sense of community in the Church. The course of these recent years shows that we are on the right path. But unfortunately, in spite of vast preponderance of the healthy and good forces of the clergy and the faithful, abuses have been committed and liberties have been taken in applying the liturgical reform. The time has now come definitely to leave aside divisive ferments, which are equally pernicious on both sides, and to apply fully, in accordance with the correct criteria that inspired it, the reform approved by Us in application of the wishes of the Council." (Alloc . Gratias ex animo, June 27, 1977: Teachings of Paul VI, XV , 655-656, in Italian 662-663).
by this present Constitution, which will be valid henceforth, now, and forever, We order and enjoin that nothing must be added to Our recently published Missal, nothing omitted from it, nor anything whatsoever be changed within it under the penalty of Our displeasure.
Furthermore, by these presents [this law], in virtue of Our Apostolic authority, We grant and concede in perpetuity that, for the chanting or reading of the Mass in any church whatsoever, this Missal is hereafter to be followed absolutely, without any scruple of conscience or fear of incurring any penalty, judgment, or censure, and may freely and lawfully be used. Nor are superiors, administrators, canons, chaplains, and other secular priests, or religious, of whatever title designated, obliged to celebrate the Mass otherwise than as enjoined by Us. We likewise declare and ordain that no one whosoever is forced or coerced to alter this Missal, and that this present document cannot be revoked or modified, but remain always valid and retain its full force notwithstanding the previous constitutions and decrees of the Holy See, as well as any general or special constitutions or edicts of provincial or synodal councils, and notwithstanding the practice and custom of the aforesaid churches, established by long and immemorial prescription except, however, if more than two hundred years standing.
Therefore, no one whosoever is permitted to alter this notice of Our permission, statute, ordinance, command, precept, grant, indult, declaration, will, decree, and prohibition. Would anyone, however, presume to commit such an act, he should know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul.