Except the liturgy of ABL/SSPX was not always the 1962 Missal as stated by the Nine:
The First General Chapter of the Society, held at Ecône in 1976, adopted the principle that the Districts and the Houses of Formation should follow the Missal, Breviary, Calendar and Rubrics which were customary at that time. This decision was never rescinded or even discussed at the Second General Chapter held last year at which your successor was selected. In the case of the United States, we have always followed the Missal, Breviary, Calendar and Rubrics of our holy patron, Pope St. Pius X, which practice was sanctioned by the First General Chapter. Of late, however, an attempt has been made to force all the priests and seminarians in the United States to accept the liturgical reforms of Pope John XXIII on the grounds of uniformity and loyalty to the Society, thereby implying that adherence to the non-reformed traditional Rites of St. Pius X constitutes disloyalty.
Lefebvre was the preeminent defender of the faith against the conciliar and post-conciliar errors, certainly, but he did not concern himself much with the liturgical rot he himself was raised upon, and which by the time of the Council was considered normal, and even traditional (eg., dialogue Masses, etc). The preconciliar damage was much mitigated by popes who were still doctrinally orthodox, and did not draw much attention (outside the revised Holy Week rites, anyway).
But in his own seminaries, we were taught -and this was the central point of the entire Liturgy I class- that by 1920, the principles animating the liturgical reform were no longer Catholic.
Why then embrace their results?
It was certainly not because of any intrinsic merit of the reformed rites and/or transitional missals, but simply a show of good will to Rome; a sign to them that though their pervasive novelties and errors necessitated a proportionately widespread resistance, nevertheless, he wanted to obey where he could, even to the point of accepting the transitional missal.
But today, we have the same conciliar authority actually permitting the traditional Holy Week, and consequently, retaining the 1962 missal has lost its purpose:
It can no longer be construed as disrespectful to authority to revert to a superior missal which that very authority now permits.
Does anyone pretend that, had Rome permitted the 1950 Holy Week/missal in 1970, that Lefebvre would still have chosen the 1962? Of course not.
Then why keep it, except as an empty memorial, to the detriment of the liturgy, and the quality of worship rendered to God?
Those who think momentum growing in the Resistance for the traditional missal indicates a slide toward sedevacantism have not understood their Founder.