#2
At that time, virtually all liturgies were identical in structure. In was in the next stage of development, from roughly the 4th to 8th centuries, that a second layer of liturgical development occurred that began the divergence that eventually produced all of the different liturgies we have today. 

The ultimate reason why the Tridentine Liturgy was replaced with the Novus Ordo, a 5th century restoration, is because when one carefully studies the entire development of the Roman liturgy through the centuries, it becomes evident that ON INTERNAL PRINCIPLES the Roman liturgy began to acquire theological unsound accretions from the 5th on wards. In my opinion, this was largely due to the collapse of theological learning in the West that began with the barbarian invasion of Rome.

There is no way to these unfortunate accretions for what they were, than by carefully walking though the development of all liturgies over time. Scholars at V2 had that perspective, and the bishops accepted it.