Just had some thoughts, for what they are worth. While it is true that yes, cantus can be translated as song as well as chant, there are some observations to be made:
1. If I am correct, the rubrics and the GIRM will be printed in the MIssal for the first time in the new translation, with all of the post-Redemptionis Sacramentum adjustments. There were editorial comments in the last Sacramentary that approached rubrics, or at least so it seemed. (Before the Syllabus of Errors gets started on this point, please remember that I haven’t said an English OF in so long, I can’t remember, and I don’t have the book here to look at it.)
2. The changes in the Missal might mean that priests who have never read the red bits or the GIRM before (and they are legion) might just do so for the first time, or for the first time since 1970. That alone might get some thinking.
3. While it may seem that the last option is still an argument to allow hymns at Mass, I am not sure it really is, whether it is in the original Latin rubrics, or the new corrected English version. Why? In any translation, it i necessary to go back to the intention of the author when using that word. Now, I would be interested if anyone could go leafing through Bugnini’s Reform of the Liturgy, his apologia for the liturgical reform, and see if he had any intention of vernacular hymns replacing the propers. From what I remember from what I have read, the answer is no. Also, if Musicam Sacram is still the proper legislation for music in the Roman Rite, is cantus as used in that document referring to the Propers?
The REAL question is this: What is the mind of the Church about music at those times? I don’t think there is any indication in any official document that the mind of the Church was to use vernacular hymns when the Propers are called for. To argue that, because hymns are used by the People of God at Mass, that is the mind of the Church and it is as such expressed in the GIRM I find a little backwards. It seems to me that the use of vernacular hymns was tolerated in the German-speaking world. Toleration in the official documents does not mean that the Church desires it for her worship, at least not universally.
4. While it is clear that hymns at Mass and Low Mass have occurred and continue to occur throughout Church history, it also seems clear to me that the Church’s intention is to SING THE MASS (Ordinary, Responses and the Propers that are in the text of the Missal) and not to SING AT MASS (paraphrases of the Ordinary as in German and Spanish, and Hymns at Mass.)
Those who are derided as having an ideology to push the propers have a better argument that trying to legislate the Propers into existence everywhere. The argument is simple: SING THE MASS, DON’T SING AT MASS!