Sadly, the reasoning is likely laced in the politics of the time. Pius XII, the first pope of the media age, filled every seminarian and priest's newspaper and television, always wearing the tiara and uttering the pontifical blessing from his lips. Papa Pacelli embodied the outward immutability of the baroque Church, if, of course, one ignores what he actually did. Pope John's Council and Pope Paul's New Mass, as Michael Davies entitled both of those phenomena, broke the baroque statue of the Church and left those reared in the eccentric obedience of the time grasping for a means of understanding what had just happened. They latched on to Pius XII as a contrast to the changes of Paul VI, true change versus false change, true renewal versus false renewal.
On closer inspection, these aspirations are quite facile. Giovanni Battista Montini, not Siri of Genoa, was Pius XII's protege and handpicked successor (one does not get rid of a priest with an under-secretary job by making him archbishop of the prior pope's see and instantly papabile). The same people who accomplished Paul VI's reforms also accomplished Pius XII's and were in fact hired by Pius XII (he personally found Bugnini). At a time when the pope was too sick to hold a consistory or meet with bishops, he was well enough to get regular reports on the progress of liturgical changes. All this will be familiar to regular readers, but to newer ones this may be a surprise.
There stands another possibility, however, that early proponents of the post-War Liturgical Movement, disappointed with the Pauline liturgy, projected their hopes onto the liturgy of Papa Pacelli. Louis Bouyer, whose arrant disillusionment with the reform would startled modern day conservatives, recounts that in the 1940s, before it was legal he celebrated the Holy Saturday Vesperal Mass after nightfall. The Paschal vigil Mass obsessed liturgists, who, far from acknowledging that its daytime celebration was a pastoral accommodation, saw the morning or noontime vigil as a great corruption, a departure from the ancient practice. They welcomed Pius XII's experimental vigil in 1951, and the next one in 1952, and the third one in 1955 (four Masses for one day in four years!). In reality, the "vigil," as it existed as a distinct ceremony from the all-night Paschal celebration of the first millennium, never took place at night; its creation separated it from the Resurrectional liturgy and put them both in the daytime. However, those who wanted to return to the ancient praxis as best they knew it may have seen in Pius XII's Holy Saturday a nod toward the primitive practice devoid of Paul VI's textual deviations.
Rather than asking "What is true reform?" Catholics would better use their time by asking "What is true tradition?"