It's interesting to compare and contrast Trent, Vatican I, and Vatican II.
i) Trent was a rearguard action. Post-Reformation Catholicism was like a post-colonial Empire. The remnants of the erstwhile empire.
When colonies or satellite countries break away, they effectively redraw the political map. The new borders of the erstwhile empire are drawn from the outside. Its borders were pushed back by the loss of its colonies or satellite countries.
At Trent, Rome allowed itself to be defined by Protestants. It ratified by boundaries drawn by Protestants. Rome was whatever the Protestants were not, and vice versa.
Whoever erects a fence first draws the boundary for both sides. By the time Trent was convened, the Reformation was irreversible. Trent was simply an acknowledgement of the new status quo. An admission of defeat. A forced accommodation to what it could not change.
Trent was not itself disruptive. The disruption had already occurred. At Trent, Rome was cutting her losses and conserving what remained.
ii) Vatican I was a one-man ego trip. Unlike Trent, which was necessary, Vatican II was elective.
Although it wasn't terribly damaging, it proved to be an embarrassment to the papacy. Problem is, the pope claims to be infallible under vaguely specified circumstances, but he rarely dares to exercise that alleged prerogative, for the moment he makes a testable "infallible" proclamation, he will disprove his infallibilist pretensions.
It's not coincidental that this prerogative has only been exercised twice since Vatican I, and on both occasions to proclaim safely unfalsifiable dogmas. The pope might as well issue an infallible encyclical on the mating habits of unicorns. You can't disprove it.
iii) Vatican II was very disruptive. And on the face of it, this was an unforced error. I don't know why John XXIII convened it. Not beyond the catchphrases about "aggiornamento" and "throwing the windows open to let in the fresh air."
One possible interpretation is that John XXIII was like Gorbachev. His Russian counterpart understood that the Soviet Empire was militarily and economically unsustainable. In that situation, you have two options: you can just let the empire fall apart–like the Roman empire and the Ottoman empire. Or you can take the initiative.
Either way, there will be losses. But if you take the initiative, you have more control over the outcome. If, by contrast, you simply wait for the empire to crumble on its own, you will be entirely at the mercy of events. Others will dictate the end-game.
Maybe John XXIII thought the Tridentine/anti-modernist paradigm was unsustainable, and he wanted to get out ahead of the inevitable break up. Indeed, even under his predecessor, the papacy was making tactical concessions to modernism (e.g. Humani Generis; Divino afflante Spiritu).
One problem with that attractive interpretation is that John XXIII isn't reputed to have been much of a thinker. Perhaps, though, the impetus came from theological advisors. In the council itself, modernism was well represented among an influential contingent of bishops and their perti. Even Joseph Ratzinger was originally a progressive theologian.
But at Vatican II, Rome lost her balance, and has yet to right herself. But perhaps, had she tried to maintain the Tridentine/anti-modernist paradigm, that would have run aground. When the fundamentals are unsound, there's only so much you can do to postpone disaster.
