Two money quotes from Henry Sire on the Mass of Paul VI and Annibale Bugnini

Pope Paul VI with Secretary of the Commission on the reform of the liturgy, Annibale Bugnini
Henry J.A. Sire, Phoenix from the Ashes: The Making, Unmaking, and Restoration of Catholic Tradition(Angelico Press, 2015):

The story of how the liturgical revolution was put through is one that hampers the historian by its very enormity; he would wish, for his own sake, to have a less unbelievable tale to tell. The partisanship in choice of agents, the contempt for law and consultation, the blind support given by Paul VI despite every abuse, the silencing of the Church's official organisms for the liturgy, the spirit of conflict in which the reform of the most sacred possession of the faithful was carried out, the advance of irreverence and impiety, the prompt discarding of principles that had been declared essential only a few years before, the discrediting and sudden departure of both the men to whom Paul VI had entrusted the reform of the liturgy, all these challenge belief. Moderation seems to demand rejection of such a story; but moderation is the wrong lens through which to judge immoderate events. That the reform of the Church's liturgical life should have been bound up with such violations seems too hard to accept, but it can be explained by two facts: the first is the initial decision of Paul VI to hand over the reform to the most extreme wing of liturgical iconoclasts, and the second is the background of Modernist clamour that existed at the time. However they chose to act, the pope and his nominees needed never to fear criticism for actions that made for change, but only for laggardness in promoting it. This noisy chorus, claiming to be the voice of the faithful, represented a milieu filled with arrogance towards the sacred and towards Christian tradition. At their demand the religious treasure house of centuries was destroyed, while the ordinary laity, under the flood of innovation, lapsed from the Church in their millions. One day it will be necessary for the Church to study with honesty the way in which its liturgical heritage was done away with and to pass the judgment that it has pronounced in the past on grave deviations from its true nature and duty. (p. 251-2)

We need to be clear that in attempting to stamp out the traditional liturgy of the Church, Pope Paul VI and the hierarchies of the world after him were following a policy of complete illegality. This assertion is not a legal quibble; it does not rest on a benign oversight in the constitution Missale Romanum. Paul VI did indeed want to consign the traditional rite to oblivion, but he knew that he was not entitled to do so. Yet even the legitimate intentions of legislation need to be expressed in legally valid form, and where the intention is legitimate there is never any difficulty in ensuring that. The failure of Pope Paul VI to abrogate the old liturgy is the consequence of the fact that it was a wholly illegitimate intention. This is merely part of a wider truth, that the entire liturgical reform is steeped in illegitimacy and illegality from beginning to end: the assumption by Bugnini and his associates of a mission beyond what the Council had authorized, the disregard that they showed for the Congregation of Rites, the ignoring of due process in the introduction of reforms, the overriding of the Synod of Bishops when it opposed the new Mass, the forcing of the new rite on the Consilium by Bugnini on the plea that it was the pope's personal will, his disobedience of the pope's direction to submit the General Instruction to the Congregation for the Doctrine of the Faith. When the new rite was brought in, the attempt to accompany its introduction with the abolition of the old was part of the same course of illegality. Hence we ought to recognize what the genuine law of the Church is at present: there is no need juridically for the restoration of the traditional rite. The only thing needed for its recovery is that the Church should return to legality. As a matter of law, there is no obligation on any priest to use the Missal of Paul VI for any celebration, and the only liturgy that has universal right in the Latin Church is the one decreed by Pope St. Pius V in the bull Quo Primum. (p. 286)

[Hat tip to Sir A.S.]