"Non-essentials matter."
"In essentials, unity, in doubtful matters, liberty; in all things charity." Pope John XXIII, Ad Petri Cathedram and popularly attributed to St. Augustine
What is non-essential for the validity of the Mass is often essential to liturgically express the faith. The idea of propitiation is an essential doctrine of the faith. But its absence in a liturgy would not render the Mass invalid--though it would render it offensive.