You have to look this up to see the error. Section 116 famously said that Gregorian chant is to have first place. But to get to section 123, you have to move past the section on music and here you discover that the sage statement about style concerns architecture and furnishings, not the core music of the Roman Rite.
The Church has not adopted any particular style of art as her very own; she has admitted styles from every period according to the natural talents and circumstances of peoples, and the needs of the various rites. Thus, in the course of the centuries, she has brought into being a treasury of art which must be very carefully preserved. The art of our own days, coming from every race and region, shall also be given free scope in the Church, provided that it adorns the sacred buildings and holy rites with due reverence and honor; thereby it is enabled to contribute its own voice to that wonderful chorus of praise in honor of the Catholic faith sung by great men in times gone by.We think here of the many Churches in Europe that were converted from the Gothic to the Classical style during the Renaissance (changes that were truly tragic in retrospect). I'm thinking too of the Art Deco at the Loyola University chapel or the Byzantine style of the National Shrine or the modernism of the Oakland Cathedral. All of these are admissible and signs of life and change in art. Rome has no set of blueprints for buildings, no stack of approved patterns for vestments, no molds for statues that everyone must copy. It true to some extent in music, as motets and Mass settings reflect the style of the times (Haydn vs. Palestrina vs. MacMillan). This are always subject to change.
But Gregorian chant is not a style. It is not music that is identified with a particular time or place or people. It is the foundational music of the ritual itself, the music that has lasted throughout the whole history of the rite. It can be substituted with another form but its status as the core, the model, the ideal, never changes. This in fact is what is meant by the seeming proviso "all else being equal" - it means that even if circumstances change that merit some other approach, the status of the chant as the number one form of music is unchanged.
But here we must consider that there is a reason why the Church put this section on changing art styles in the section under architecture and furnishings. It is precisely to avoid the confusion that chant can be entirely displaced. Gothic styles and Art Deco styles can be entirely displace; Gregorian chant cannot be, which is why section 116 says what it says. This was a major contribution of the Second Vatican Council: to settle this issue once and for all.
The upshot of Fr. Ruff's article is to argue that if we take Gaudium et Spes seriously, we must be open to modernity and adapt our ways to fit it. However, I find nothing in Gaudium that would unseat Gregorian chant from its primary place in liturgy. No, chant does not make Mass a "museum piece" any more than reading the Gospel means that we are somehow stuck in the past. The Gospel and liturgical chant are timeless things.
I really do not understand why people have such a difficult time understanding these distinctions, but apparently this confusion is common. I receive many emails from people who are somehow under the impression that this blog is all about promoting our personal taste and displacing the personal taste of others. Again, the opinion here is not unlike what Vatican II says: there are certain features of liturgy that are beyond taste, and chant is certainly among them.