Palestrina,

No, you're not alone in encountering this opinion.

The question of how to dismantle it systematically will depend partly on what your environment is.

Begin from the assumption that the Sung Mass (rather than the Spoken Mass) is normative (which in this context means "the measurement according to which other questions are answered.)

Since the Sung Mass is normative, the Church provides us, in her official books, both the texts to be sung and the melodies with which we should* sing them.

[If you live in OF-controlled territory, the argument has to begin elsewhere, because the foregoing is not held to be true]

[*The Church allows music which is other than the chant to be sung for the assigned texts, and this requires further development in a moment]

Among the chants to be sung at Mass, some are more intricate and others are less intricate. Psalm verses at the Introit, for example, are frequently simpler than the Introit antiphon itself. Gloria Patri, where it exists within the required texts of the Mass, is frequently the easiest of all to sing.

Even within the same Mass Propers, some are more intricate and others less so.

Among Mass Ordinaries, some are more elaborate and others simpler.

The fact that the Church provides music of varying degrees of difficulty, some of which is beyond the skill of untrained singers argues in favor of expecting the trained singers to do that which the Church expects, and thus that the choir is not merely leading the assembly. This is a question which does not depend on the era in history , or the age of the congregation's members or any such thing.

*Let it be acknowledged that once one is singing the prescribed texts, the Church gives generous application to the idea that music which can bear the weight of the august ceremonies which exist for the public worship of God may be admitted to the Mass (and the rest of the Office). Accordingly, more intricate polyphonic settings of the Propers may be used, and so may simple psalm tone Propers' settings, if there is a genuine need for such things. The identification of the "need", however, isn't based on the tastes of the assembled faithful, lay and clerical, but on the skill of the musicians.

Yes to the GIRM.

In my experience, this opinion is born of a more fundamental error, namely, that those who exercise liturgical functions are somehow 'privileged' or have in some measure 'taken on authority'. The perception, then, is that when musicians claim a liturgical role, they are appointing themselves to the 'pleasures' of this privilege or authority.

Of course, playing a liturgical role (of any sort) is first and foremost a service, and while I would be the first to say that I feel privileged to be able to serve in that capacity, it is, fundamentally, a service.

The above is just an observation. I'm afraid I don't know how to clarify the situation for others. The problem is that saying what I just said to the wrong person sounds again like so much self-aggrandizement. My own approach is to say very little and act with humility while unapologetically serving the role that I am called to serve.

I think it wins the argument, whether the other person concedes or not. Once you cite the GIRM, then the person who contends otherwise knows that he's not arguing against your or my personal opinion, but against the Church's decision.

The point about the choir's distinct role is stated explicitly in plain language. In the other case, the relative inferiority of "alias cantus aptus" has to be inferred.

NB alius cantus aptus has been changed to alius cantus ... congruus.