Reform of the mutable aspects of the liturgy* has occurred throughout the Church's history. Pope St. Pius V standardized the Roman Rite in his 1570 bull Quo Primum, and Pope St. Pius X reformed the breviary in his Divino afflatu. John XXIII was the first in 1½ millennia to change the Roman Canon prayers of the Mass when he added St. Joseph's name to it.
For there to be a valid Mass, the form of consecration ("For this is my body…" and "For this is the chalice of my Blood…") cannot ever change. Defects that occur in the Mass (which may or may not render the Mass invalid) are summarized in Pope St. Pius V's De defectibus.
That the liturgy must "adapt to the cultures of diverse peoples" is a novelty since Vatican II. Liturgical reform prior to Vatican II has always been about better conforming the liturgy to God's liturgical law, transmitted via tradition through the Church. Just as the Old Testament Jews had ceremonial precepts* in which God prescribed exactly how they are to worship Him, so too does the New Law include liturgical law.
*Ex. 32:17-19 (false worship of golden calf) shows how angry Moses got when the Jews violated liturgical law. They had good intentions (they thought the calf was a likeness of God and that they were honoring God by worshiping it), but they violated how God commanded they worship. They were so irreverent (shouting, singing, dancing) that Aaron thought there was a battle going on!