Exposition and Refutation of the Principal Post-Conciliar Doctrines
I believe that the doctrines which emerged or were imposed after the Second Vatican Council constitute a rupture with previous Catholic teaching. I do not regard them as mere pastoral developments, but as doctrinal novelties incompatible with the traditional Magisterium. Here I set out the principal ones, the reasons why I reject them, and their practical implications.
VII was a pastoral council in response to the modern era (~1700–mid-20th century, enlightenment, reason, science, industrialization, individual rights, belief in progress and objective truth). Nothing new binding on our conscience. We are now in the post-modern era (~1960s–present, skepticism toward universal truths and institutions, pluralism, relativism, emphasis on identity, language, perspective and social construction). It's no wonder that the youth prefer Tradition and the TLM.
Your distinction between modernity and postmodernity is interesting, but it does not fully explain why so many young people are turning toward Tradition. And above all, saying that Vatican II brought “nothing new binding on our conscience” is an oversimplification.
The Council did present itself as pastoral, rather than as a dogmatic council solemnly defining new dogmas. But its documents nevertheless contain doctrinal teachings and orientations that profoundly affected the Church: religious liberty, ecumenism, collegiality, the understanding of Revelation, liturgical reform, and so on. To claim that there was “nothing new” is therefore difficult to defend.
And if so many young people are turning toward Tradition and the Traditional Latin Mass today, perhaps it is not simply because they love the “past,” but because they are searching for continuity, transcendence, clear doctrine, and a liturgy that does not seem to have been shaped by the spirit of the world.
The real issue, then, is not simply “Vatican II versus the modern world,” but something much more fundamental: how do we distinguish legitimate development of Tradition from a rupture with it?
Words mean things. I would argue that modernism, "the synthesis of all heresies", shares a root cause. What is it? Well, let's study Satan. During the 2nd temptation, Satan tells Christ to jump off the temple, and quotes scripture "the angels will catch you". He is quoting Psalm 91:12 which states that "In their [angels] hands they shall bear thee up: lest thou dash thy foot against a stone." If you read Pslam 91, angels catching you is predicated on fighting evil, not creating a David Copperfield show. Satan changes meaning. Those under the influence of Satan change meaning. Every heresey is an attempt to change meaning. How do we protect ourselves from Satan? Catholics are originalists. That is, when reading text, we ask the question "what did the original author intended the original reader to understand." Satan and those under the influence of Satan are non-originalists, such as textualists or pragmatists. All the VII council fathers took an oath against modernism, which is an oath against changing the meaning of Tradition and Scripture. Consequently, every word in VII must have an originalist interpretation. Modernrists changed the meaning of "religious liberty, ecumenism, collegiality, the understanding of Revelation, liturgical reform, and so on". No surprise, they despise Latin, a stable language that preserves meaning. The unpublished text of the Third secret of Fatima (the vision was published in AD 2000 but not the text), warns of Satan rising to the top of the Church. Returning to your question "how do we distinguish legitimate development of Tradition from a rupture with it?" Answer, return to originalist interpretation of all text, and everything else is from Satan. The youth, really, are searching for original teaching, and liturgical expression thereof. Godspeed.