Pope Pius XII, and others no doubt, didn’t see the Liturgical Movement as a universal good. MD 7 suggests that this encyclical is written, in part, to ensure it keeps to the straight and narrow.
7. But while We derive no little satisfaction from the wholesome results of the movement just described, duty obliges Us to give serious attention to this “revival” as it is advocated in some quarters, and to take proper steps to preserve it at the outset from excess or outright perversion.
Perversion is a strong term with something of a different overtone in the 21st century Church. A bit more elaboration on this point:
8. Indeed, though we are sorely grieved to note, on the one hand, that there are places where the spirit, understanding or practice of the sacred liturgy is defective, or all but inexistent, We observe with considerable anxiety and some misgiving, that elsewhere certain enthusiasts, over-eager in their search for novelty, are straying beyond the path of sound doctrine and prudence. Not seldom, in fact, they interlard their plans and hopes for a revival of the sacred liturgy with principles which compromise this holiest of causes in theory or practice, and sometimes even taint it with errors touching Catholic faith and ascetical doctrine.
Less than two decades later, the enthusiasts were in ascendancy. I’m not sure what to make about the “taint” mentioned above. It might be that the stasis of liturgy after Trent introduced or even maintained certain errors as well.
9. Yet the integrity of faith and morals ought to be the special criterion of this sacred science, which must conform exactly to what the Church out of the abundance of her wisdom teaches and prescribes. It is, consequently, Our prerogative to commend and approve whatever is done properly, and to check or censure any aberration from the path of truth and rectitude.
What seems missing here is the integrity of faith and morals in light of the Gospel mission of Christ. What is fruitful for the spread of the Gospel? That seems to be the next stage taken by the conciliar bishops in 1963 and the course we’ve struggled with in the half-century since.
A balanced approach that chides those inclined to infighting:
10. Let not the apathetic or half-hearted imagine, however, that We agree with them when We reprove the erring and restrain the overbold. No more must the imprudent think that we are commending them when We correct the faults of those who are negligent and sluggish.
Negligence by not attending to the celebration of liturgy with quality: good words for 1947 and for today, too.
Remember to check Mediator Dei on the Vatican web site.