It is certain that the sudden drop in Mass attendance and the overall loss of faith were caused by a very complex combination of factors. The factor of cultural craddle Catholics "angered" by Humanae Vitae can be neglected, as especially in the decadent western countries most clergymen simply ignored Paul VI's encyclical and continued to support dissident opinions. At least in France, the Netherlands, Spain (after 1975), Germany (after 1968) and England this was sadly the case.
The huge drop in Mass attendance however happened during the Council already and the ten years after it.
By the cultural liberal and cultural-Marxist revolutions which swept western societies, the youth was incited to "cast of" religious affiliation and things considered mortal sins at once became "hip", sadly enough many teachers, student unions and even many religious sisters and priests accepted those positions and some even left their posts for that Revolution.
At the same time for the more serious Catholics, as opposed to the cultural pillar Catholics who increasingly were alienated by them being indoctrinated into the new revolution's morality and religious opinion, were increasingly confused and confronted with theological timebombs and atomic bombs in Vatican II and the "Catholic" media. All of a sudden church leaders "de-affirmed" the dogma of Outside the Church no Salvation, of the existence of eternal hell, of the necessity of confession, of the only way of salvation being Our Lord Jesus Christ. Then shortly after those inner theological confusions came the big tsunami wave of the liturgical reform "in the spirit of Vatican II". The center of their faith was done away with and reformed to a political human self-affirmation exercise at worst or a protestantized ever evolving and changing Mass at best.
The constancy was lost and in it many faithful, like thousands of priests and religious brothers and sisters who apostatized and joined the cultural revolution (prepared for it by theological modernism and political infiltration since the late 1940s), lost their Faith and Faith practice.
The serious Catholics just sought other ways, as the pluralist and anti-dogmatic "evolutionary truth" concept sold to them as being the teaching of Vatican II, while being affirmed not morally but certainly theologically by a liberalizing and ecumenizing Vatican apparatus, is not acceptable to a logical, rational mind of a serious human person seeking eternal truth - which by nature must be unchangeable.
Remember: most bishops were replaced with "modern" and "contemporary pastoral" ones and many priests were confused after the Mass was taken from them by those supposed to guard the treasury of the Church. All went within 10-15 years.
At the same time wealth and growing welfare systems took away existential fears and questions which normally lead people to seek meaning in life and afterlife, whereby the quest for a still authentically Roman Catholic bishop was not as attractive to western-decadent "ever more alienating" Catholics as it was for French Catholics during the 1789 and 1793 Revolutions.
Remember, in the early 1970s Ecône was very far away, especially for Spanish, Mexicans, South Americans, yes even for Dutchmen and Italians and Englishmen. After the rise of it after 1976 the greatest damage was already done. The elderly with a steadfast Faith often lacked mobility and financial means to support the traditional movement. And Rome in some countries wished to "take away" the "Lefebvrian sentiment" among some by appointing "conservative" Novus Ordo bishops, like in Roermond Bishop Joannes Gijsen, a Dutch bishop currently bishop of Reykjavik, Iceland.
This is how things developed. The remnant of Roman Catholics was again divided over the liturgy, and even among traditionalists among them, there were huge conflicts over phony and fraudulent "apparitions" (apparitionism) like those in Palmar de Troya, Medjugorje, San Damiano etc. etc.
The uncertainty and the revolution inside Rome and the Holy See were the main causes for the confusion and subsequent falling-away of the faithful. Also the dissolution of eternal truths by local bishops and priests dedicated to the "Reform" (=Revolution).
Humanae Vitae merely affirmed what was already Roman Catholic teaching since ages. Even HV was more liberal than previous regulations, as it also permitted Natural Family Planning for minor causes, forbidden previously. It was turned by liberal "Catholic" activists "inside" the Church and parish structures into a symbol against that which was in need to be despised and combatted against by demonstrations and activism, but in the eyes of the average Catholic, it was of minor importance - especially as in the west many priests continued to support the anti-birth pill and condoms, at least tacitly.
An important factor might have been after 1968 that anti-Catholic myths, black legends and crypto-socialist atheist propaganda was introduced into the media and especially at schools, which caused another reinforcement and support for the already large confusion and existential doubt among many (former or in the process of apostatizing) Catholics.
The elderly who were not exposed to these indoctrinations of the liberal/leftist cliques however rapidly are dying out. They continued to attend Masses, even if very bad, out of custom and because of their fear to be disobedient to God and the Church. They just did not swallow everything and they did not internally support the liturgical revolution, by the hierarchical system of the Roman Catholic Church of all the ages before - which not even allowed calling into question personal actions of priests sadly - prevented them from starting a restorative movement in all parishes. They just went along externally, not internally.
The internally spoilt and ruined generation is that of those born in the mid 1940s and the late 1940s. The later generations simply do not know the Faith anymore, or only a caricature or some home or granny remnants of it. But the 1940s children once had the Faith, but lost it during the revolutions.