Vatican II simply contradicted the immemorial teaching of the Church prior to it; this is, at a minimum, an historical observation, and not merely a theological one. Take the Limbo of the Children, for example. No theologian or even layperson ever denied the existence of such a place, where those individuals who end their lives "in original sin alone" spend the rest of Eternity. Of course, the whole idea of "original sin" has been completely abandoned in the post-conciliar Church within its "lip-service theology." Since post-Vatican II "Catholicism" contradicts itself (by not affirming that which was always affirmed) and continues to "change" on teachings which were affirmed as being unchangeable, it cannot be a source of Truth and must be abandoned.
As for atheism, it is at least a consistent philosophy, albeit a false one. Matter & Energy is all that there is and all that there was and all that there will be; nothing else. No God and no afterlife. Atheism is at least consistent with itself, and therefore, should be at least preferable to a religion which contradicts itself. (And, no, I am not about to become an atheist!)
On the other hand, traditional Catholicism is at least consistent within itself, in that Vatican II taught and promulgated error; as such, traditional Catholicism affirms that which the Church has always taught.