Go to a monastery and - maybe - they are singing vanilla liturgies.

The "vanilla" liturgy proposed by the original poster is, sadly, not a vanilla liturgy in most of the U.S. Many, especially those of a certain generation, would react if the spice were horseradish.

If all the OP were saying was:

This music is complete. It's beautiful. It's in the book.

then he would be standing on good ground, since St. Pius X agrees,

The ancient traditional Gregorian Chant must, therefore, in a large measure be restored to the functions of public worship, and the fact must be accepted by all that an ecclesiastical function loses none of its solemnity when accompanied by this music [i.e. Gregorian Chant] alone.

or indeed, if the OP also said:

Less speaking. More singing!

,

Then we could point to Paragraph 113 of Sacrosanctum Concilium:

Liturgical worship is given a more noble form when the divine offices are celebrated solemnly in song, with the assistance of sacred ministers and the active participation of the people.

But I find it harder to square:

Toss the hymns. Toss the anthems. Toss the motets.

With the attitude that St. Pius X tells me I should take towards other forms of music in the liturgy:

The Church has always recognized and favored the progress of the arts, admitting to the service of religion everything good and beautiful discovered by genius in the course of ages — always, however, with due regard to the liturgical laws. Consequently modern music is also admitted to the Church, since it, too, furnishes compositions of such excellence, sobriety and gravity, that they are in no way unworthy of the liturgical functions.

Or, indeed, the understanding of my vocation as a Catholic organist and composer which paragraphs 120-121 of Vatican II's Constitution on the Sacred Liturgy enjoin me to embrace:

In the Latin Church the pipe organ is to be held in high esteem, for it is the traditional musical instrument which adds a wonderful splendor to the Church's ceremonies and powerfully lifts up man's mind to God and to higher things.

Composers, filled with the Christian spirit, should feel that their vocation is to cultivate sacred music and increase its store of treasures.

The OP writes of what I do as a Catholic choirmaster (programming hymns, motets, pet projects, etc...):

Why does everyone complicate it so much?

But what I do when I do those things is not complicating things.

It is sharing the priorities and vision of the Church as expressed over a century of Magisterium.

Priorities both grounded in the deepest part of our musical heritage, the Gregorian Chant, but which are rounded out by an openness to beauty and a recognition of the riches the Church possesses in centuries of music written for Her.

Put another way:

Liturgy which is...

(1) primarily sung,

(2) with a beating heart of plainchant that is a unifying musical fabric,
on the voices of the choir for what is proper to them,
and in the hands and on the lips of the whole people of God,

(3) beautified and supported by the pious playing of the organ
at appropriate times and seasons, and

(4) enriched by the fruits of the progress of the arts across centuries,
in particular the flower of the polyphonic school,

(5) but without excluding the music of later times
and of our own day which exhibit due artistry and are suitable,

(6) embracing fruits of a progress which is recognized and promoted by the Church,
and to which those with the requisite gifts should take it,
as their vocation, to contribute,

(7) respecting, promoting, and tying the devotion of the people
to its source and goal, the sacred liturgy,
through the judicious use of vernacular hymnody.

...that is simplicity itself.

On the other hand,

Setting yourself up to explain to a man of simple and deep faith why you will not suffer him to sing "Hail, Holy Queen" after a feast day Mass of Our Lady,

or to your singers why Palestrina's Alma redemptoris is insufficiently "vanilla" for inclusion in your parish liturgy,

in spite of its beauty, dignity, and accessibility without real risk of screwing it up,

and despite Palestrina himself & his art being expressly singled out for commendation by the highest authority in the Church across multiple pontificates...

that is complicating things.

If you don't believe me,

try it.

The vanilla liturgy, while sufficient, is also a foundation upon which to build. Master it, and subsequent musical additions are put on solid footing. Buxtehude's Thaxten-variationen is a wonderful set of compositions. And for sure they can enhance the Mass. But in my experience such things nearly always REPLACE the Mass. Sure, "Hail, Holy Queen" is a nice devotional addition. But if we're singing this or Palestrina, while leaving such as the creed, the propers, and the dialogues unsung, I think the musical priorities are in need of reform. I desperately thirst for the liturgical basics. I love vanilla liturgy! But church musicians are too busy showcasing liturgical additions, at the expense of the liturgy itself.

Question for every music director of every Mass I've ever attended: Why do you all spend BY FAR the bulk of your time and energy on music that is accessory to the Mass, while the Mass itself is musically neglected?

...and the dialogues unsung...

Nothing is stranger than beautiful, complex music juxtaposed with an entirely spoken Mass. It's like wearing a tuxedo with flip-flops.

Another brief thought -- I had exactly this conversation with a parishioner recently...

It is *because of* the fabric of the chanted Proper and dialogues that Motets etc we do don't feel out of place or out of proportion. The chant makes everything at home.

Interestingly enough, after the closures of churches for a half a year, the only two churches around here that have recovered fully are the TLM church (which has probably more than double in attendance since then) and mine (which has increased slightly). My church is the one that is known as the one to attend for a reverent liturgy if you can't get to the TLM.

The rest? They lost a lot of people (and donations) and haven't recovered.

the music...is not part of the sacred action.

That's a rather bold claim. Considering there are rubrics governing specifics about the way music is done in the '62, this hardly seems accurate.

Necessitating that everything stop while the ordinaries are sung awkwardly disrupts the natural flow of the liturgy.

do not want...trumpets at Mass

Really??? Not every Sunday, but surely on occasion??

[Edited] Once SCR had been established to regulate the liturgy, they made rules, it's what bureaucrats do, usually badly. Fortescue's private comments are pretty pungent.

Necessitating that everything stop while the ordinaries are sung awkwardly disrupts the natural flow of the liturgy.

OTOH

There was something to reform and to rediscover. ...; the priest singing in the traditional melodies the Kyrie, the Gloria, the creed with the faithful; these are so many good reforms that give back to that part of the Mass its true finality.
Abp. M Lefebvre; Itinéraires vol 95 July-August 1965

However I concede trumpets on certain occasions :-)

I always assumed that practice developed with the integral Missals printed for Low Mass being used at Sung Masses, and then was codified.