The non-existent taboo against composing new Gregorian chants

It is obvious to anyone who attends a Catholic liturgy that despite repeated attempts by those in authority Gregorian chant is far from being "given pride of place in liturgical services". One problem is that it is not a form of music that can be easily and readily played by ordinary musicians. It requires specialist training. Another problem is that the chants themselves are complicated. Many of them can really only be sung by a choir. One way to overcome this was the Graduale simplex in usum minorum ecclesiarum. As its full title indicates, it was meant for the use of Churches too small to sustain a full Gregorian choir. Judging by the present situation that would be pretty much all Churches, including most Cathedrals.

According to Archbishop Bugnini, when the Graduale simplex was first presented, there were loud objections. In his memoir of the reforms he quotes an objection and answer document published by the Consilium.

II. "New forms would be introduced that are not adapted to the faithful and not in conformity with the art of the Church and with the liturgical renewal."
Answer: Not in the least! the melodies of the Graduale simplex are all in the present chant books. None of them is new. The manner of singing, in which one or more cantors alternate with the congregation, which sings a refrain verse, is the oldest and most traditional in the Church. Its use has shown how easy and possible this kind of singing is; the truth of this claim was seen at the fourth session of the ecumenical Council and can be seen every time the congregation responds to the chant with an easy verse, as often happens even in televised Masses. This manner of singing is completely in conformity with the Church's art, as is shown by the venerable tradition dating from the time of such Fathers as St. Ambrose and St. Augustine [presumably referring to Augustine, Confessions ix.7]. It is also consistent with the liturgical renewal, since one of the reform's basic principles is the active participation of the faithful in both the actions and the singing of the sacred rites. (Bugnini, The Reform of the Liturgy 1948-1975, chapter 58, p.894).
(Jeffrey Tucker discusses the Graduale simplex at the Chant Café).

This suggested to me that there is some kind of taboo against composing new plainchant melodies. I have seen it held against the Graduale simplex that it uses melodies from the Divine Office, so a fortiori one would expect there to be some rule against composing entirely new ones. It should be noted that Urban VIII's revisions of the Breviary hymns (mentioned the other day) to make them more Classical and in accordance with Classical metres, are criticised by the Catholic Encyclopedia (§VI) partly because in fact more recent scholarship has shown their Classicism to be defective. Perhaps the fear is that in composing new melodies unknown rules of composition would be broken.

On the other hand the Pope's Mass for the opening of the Year of Faith "used new compositions in the Gregorian tradition for the introit and communion".