And what happened to the faith of those attending(and saying) the Tridentine Mass, that they let the New Mass come into being in the first place?
Fr. Wathen makes a decent point, but I don't think "people who do X are generally much more lax in faith" is enough proof to say that "doing X" is mortally sinful. It's an indicator to be wary, sure, but not proof it's grave sin by any means.
The point is, like it or not, he gave a crystal clear answer, and also clearly explains the reason for that answer. He did not take the middle ground spinlessly saying "mmmm, well, it depends..." That person and many others needs a clear answer to that clear question. In typical fashion, TIA act like it is unanswerable, or a great mystery, or that it depends on the individual's thoughts and feelings.
As to your first question, it is exactly as Fr. Wathen said - it is a matter of faith.
The people willingly gave up their faith, some did so with great glee. One of my aunts for example who proudly boasted that she was the first female eucharistic minister in the country, that was around 1967ish. My folks argued with her and her husband till they were blue in the face over the whole crisis situation, and did the same with the rest of our relations, all to absolutely no avail. THOSE were times of mass confusion, but after 60 years of this crisis, Fr. Is right;
"all you need to do is consider what has happened to the faith of those who have been attending the new mass through the years". To quote Last Tradhican from another thread:
"Unfortunately, only maybe 1% (if that) of baptized Catholics live the faith". It *is* a matter of faith.