Speaking as one who was and in many ways still is one of the ‘new spring’ I can gives some comments.
I was a daily mass going, confession once a week, rosary daily Catholic and I eventually went to the seminary with a love of God and the Church. I am only 24 now, and I was in the seminary last year after completing a degree at a secular university which had an Opus Dei chaplain (we became good friends). I frequently attended Opus Dei events and while at school, organized the student body to aid the local Catholic soup kitchen (even though I was usually the only one ever to go, aside from a Philippino friend who has also lapsed in his enthusiasm for the Church).
Simply put…the enthusiasm wains. This is no different today as it was with my uncles (the end of the baby boom) who both went briefly to the seminary. One is now an Anglican and the other is (by the standards of thir board) a heretical Catholic professor of Biology at a prestiguous Canadian University.
The enthusiasm wains for several reasons for most and this is the same with my protestant friends who were all God-crazy in highschool and part of university. I would have to say that the main reason the enthusiasm drops is simply as people move from the juvenile level of Apologetics to the academic fields of History, Philosophy and Theology, it becomes evident that much the Church teaches simply isn’t true.
I used to read the apologists like Keating, Kreeft, and Madrid but I eventually (in an honest desire to learn more to better support my faith) started to read Gilson, Maritain, Rahner, Jungmann, Copleston, Chenu, Klauser, Newman, Dawson, Knowles, de Lubac, Congar,(Chesterton is still a favorite) etc. The previous simple apologetics answers were simply not adequate. Ironically it was at the seminary that a real shift in my faith occured, not because of the liberal professors who taught the modern and contemporary philosophy courses but because of the conservative religious professors who taught the Medieval philosophy, liturgy, theology and scripture courses.
My guess for the next generation of Catholics is this: some will fall away or become the much despised liberals either through laziness or true scholarship, and others will become simply the Catholic equivalent of James White fundamentalists. We see the fundamentalist trend (especially in the US) with the growth in popularity of the SSPX or even the Priestly Fraternity of St Peter (I had a friend who went there and left disgusted with the Feeneyist/Jansenism found within) while at the seminary I chose I was squeezed between the ignorant conservatives (who couldn’t manage a passing grade if their life depended on it) and the homosexuals.
I would hate to see the Catholic Church go the way of American protestantism.