On the other hand, it is true, IMO, that practically all of the present problems can be traced to modernism, which, as you know, Pope St. Pius X warned about even in 1907.
I'm no expert on Church history, but IMHO, and from what I have read on the matter, it could be fairly said that the Council, the new Mass, the air of reform to the point of excess, and the "spirit of Vatican II" merely gave modernism the room to breath that it subsequently used to inflict the harm on the Church that we now see.
The decline did not begin until the close of Vatican II. Just before the Council the Church was actually at its zenith in terms of numbers of vocations, Mass attendance, conversions, baptisms, etc. It was at the apex of its political and cultural influence--even one of its bishops, Bishop Sheen, was as popular on national t.v. as Milton Berle. The decline began precipitously within ten years after the Council's close, with the incredible--and brutal--revolution that followed. Everything abruptly changed--liturgy, devotions, the Church calendar, sacramental rites, theology, seminary training, etc. With the introduction of the Novus Ordo, Mass attendance of all Catholics dropped abruptly from 80%+ to around 25%. It now hovers at around 17%. Belief in key dogmas also dropped precipitously. Belief in the Real Presence plunged. Now only 25% of all Catholics believe in this essential dogma. So too with belief in the Resurrection. Here is how Bishop Fellay put it recently in an interview:
"The cause [for decline of faith] is within the Church, but when someone tells them that, they cry out: No, if something isn’t right, it must be the world’s fault.
But allow me a little analogy, that of the chicken coop. It’s well known that an open henhouse allows the fox to enter and ravage the place. They say, It’s the fox’s fault. And I dare say, Who left the door open? The fox does its job. It’s the same with the world, it does it’s job trying to destroy the Church and corrupt Christians. To say that it’s the world’s fault that things are not going well is the same as saying it’s the fox’s fault. I’m sorry, but a mea culpa is owed by the one who was in charge of the door. Because this time it is not a matter of negligence, but of choice. At the Council there was an opening to the world; they not only opened the door, they took it off its hinges. At the time, there was talk of opening windows. To let in a little fresh air, the Pope said. But in fact it was a whirlwind that entered. Now there’s a disaster and it’s the world that is to blame. I’m sorry, but the door needs to be put back, and quickly."