You, Pax, recognize that "custom" can include a range of practices, some of which are easily changed and some of which are not.

How is it a substantial change to the theology of the mass to say responses with the servers, or to recite something that the people sing at a sing mass?

My recollection of the Latin mass is the priest doesn't say the responses the servers say.
And the people sing the creed at a sung mass. Does that take away from the priest?

I can understand the laity (sheep)  just going along, that is why we are where we are today, but my question is directed to those who promote the mass like the ElAusente and anyone else who promotes it and fights for it. Why do they want to respond aloud like the altar server and the priest? In the case of a priest, why do they want the congregation to respond aloud like altar servers?

First, let’s review.  In a nutshell here are the degrees permitted.
The parts that could be said or sung by the congregation were of two kinds: the parts to be sung at High Mass (Pontifical, Solemn, Sung), and the parts which are responses of the ministers or the server at Low Mass.  The 1958 docuмent Musica sacra divides dialogue Masses into four degrees of outward, vocal expression.  In a nutshell,

At Low Mass

c. Participation of the faithful in low Mass.

28. Care must be taken that the faithful assist at low Mass, too, “not as strangers or mute spectators” (Divini cultus, Dec. 20, 1928: AAS 21 [1929] 40), but as exercising that kind of participation demanded by so great, and fruitful a mystery.

29. The first way the faithful can participate in the low Mass is for each one, on his own initiative, to pay devout attention to the more important parts of the Mass (interior participation), or by following the approved customs in various localities (exterior participation).

Those who use a small missal, suitable to their own understanding, and pray with priest in the very words of the Church, are worthy of special praise. But all are not equally capable of correctly understanding the rites, and liturgical formulas; nor does everyone possess the same spiritual needs; nor do these needs remain constant in the same individual. Therefore, these people may find a more suitable or easier method of participation in the Mass when “they meditate devoutly on the mysteries of Jesus Christ, or perform other devotional exercises, and offer prayers which, though different in form from those of the sacred rites, are in essential harmony with them” (Mediator Dei, AAS 39 [1947] 560-561).

In this regard, it must be noted that if any local custom of playing the organ during low Mass might interfere with the participation of the faithful, either by common prayer or song, the custom is to be abolished. This applies not only to the organ, but also to the harmonium or any other musical instrument which is played without interruption. Therefore, in such Masses, there should be no instrumental music at the following times:

a. After the priest reaches the altar until the Offertory;

b. From the first versicles before the Preface until the Sanctus inclusive;

c. From the Consecration until the Pater Noster, where the custom obtains;

d. From the Pater Noster to the Agnus Dei inclusive; at the Confiteor before the Communion of the faithful ; while the Postcommunion prayer is being said, and during the Blessing at the end of the Mass.