I have been unable to find any conciliar mandate for the post conciliar treatment of the body of Collects; treatment, in one significant respect (Sundays), decidedly more ruthless than what a Zwinglian Reformer, Thomas Cranmer, did at the height of the English Reformation.
Sunday collects. The collects for the Sundays of Advent, Lent, and Eastertide were, almost to a man, replaced. What this means is that the whole body of such old prayers was deemed inadequately to express the indoles of the respective seasons ... a breath-taking condemnation!. I can think of at least three academic studies, beginning with that of Fr Cekada, which have revealed the hidden ideological basis of this revolution (often a sort of practical Pelagianism). There is a considerable danger in such radicalism. The true mystagogue - such as Gueranger - derives his mystagogy from his studies in the euchology which eighteen centuries have handed down. He does not form his views on a priori grounds, and then take a pair of scissors to the Tradition.
As far as concerns the collects for the 'green' Sundays, 17 of the 34 are new importations.
Festival collects. The new books reveal a massive campaign to rewrite the collects for festivals of the Lord and of his Saints. This has had a particularly vicious effect as far as the survival of the older collects in the previous books is concerned. Those older collects, many of them in continual use since the days of the early sacramentaries, were commonly terse formulae whose main purpose was a desire to secure a share in the intercessions and fellowship of the glorified servants of God, especially the martyrs. In the Middle Ages, a different style of collect became dominant; one can analyse it as providing God with a biographical summary of the saint concerned, followed by a request that the worshippers might receive congruent graces. (The collects written by Cranmer for those saints who retained propers were all to this medieval formula.) The post conciliar reformers were also wholly committed to the same procrustean methodology. I do not wish to be understood as arguing for the exclusion of such collects: my point is that we should not be limited to them.
You see, my own feeling is that the body of collects, on the eve of Vatican II, found its main and exhilarating strength in its variety. As the days moved on, one went from a Leonine or Gregorian form to a Carolingian and then to a Franciscan composition, and then to a product of the Baroque counter reformation. I see this pluriformity as healthy; it prevents the Church from being imprisoned in the culture of one euchological register. Which is what the post-conciliar books give us; so that, now, all our eggs are in the basket of one particular style. Even where a 'new' collect was in fact resurrected from an ancient source, the bar it had to reach was acceptability to the 1960s ... otherwise, its only hope of being chosen was if it could be bowdlerised into such acceptability. I feel certain that the style and preoccupations of the 1960s have already proved to have dated considerably. Perhaps there was a case for replacing a few of the more plodding of the nineteenth century collects which indeed did tend to say rather obvious and pedestrian things ... I am not a fundamentalist ...
... but Vatican II gave nobody any mandate even to do that much.