The RCC is hemorrhaging members as well, in addition to evangelical Christianity in general (except maybe Pentecostals, i think). It has nothing to do with the theological or political liberalism of Mainline churches. The west is simply secularizing. It has been for decades. Eventually there will be a revival, hopefully, but who knows what that will look like. One thing is for sure: young people aren't interested in religious legalism, bigotry, or fire and brimstone.
Alas, that’s simply not the reason for the loss of members. If you look at traditional Latin Mass parishes, they are filled with families and young people. Every pew is full at the local TLM, and this has remained the case despite Traditiones Custodes (and indeed, I would note that there have been no traditional latin masses in this area suppressed as a result of Traditiones Custodes, so one cannot attribute the growth in attendance to a consolidation of available Tridentine masses in the area - it is rather authentically organic).
Likewise, the Orthodox Church has been experiencing a spectacular influx of converts, and also has a high reproductive rate. The only Eastern Orthodox church in North America that is contracting is the Greek Orthodox Archdiocese, which has a reputation for being somewhat liberal and ethno-centric, and in general those who are not ethnically Greek have historically had less fruitful careers with GoArch than those who are, although in recent years this has fortunately been changing.
On the other hand, the Coptic Orthodox Church, the Syriac Orthodox Church, the Antiochian Orthodox Church, the Orthodox Church in America and ROCOR, among others, have been seeing a very steady influx of converts as well as new arrivals from the Old Country in the case of the Copts, Antiochians and Syriac Orthodox (who live largely in those areas of the Middle East which have become increasingly hostile to Christians in recent years). And the reproductive rate among both groups is high, with frequent baptisms of newborn children, and frequent receptions of entire families, moreso into the Eastern Orthodox churches, but the Oriental Orthodox, especially the Coptic Orthodox, are seeing an increase in conversions.
Likewise, I have heard that some of the more traditional liturgical Protestant churches, such as certain Continuing Anglican churches of the Anglo-Catholic persuasion, and Evangelical Catholic Lutheran churches, which emphasize a traditional liturgy and traditional moral values, are benefitting from this same conversion and high reproductive rate.
The reality is that traditional Christians will reproduce at higher frequencies, and the younger generations of Christians seem more interested in liturgy and mystery than older generations. This should have been obvious given the liturgical aspects of Taize in Europe, but this fact was missed, and instead a very watered down liturgy was implemented across the RCC and mainline Protestantism, one which stressed “active participation” and “engagement” over silence, mystery, contemplation and liturgical beauty. And the result is an experience which is overly intellectual and rational, and lacking in opportunities for prayer. There is a reason why Eucharistic Adoration and the public prayer of the Rosary and the Novena is so popular among Roman Catholics - they provide mystery, silence and the opportunity for contemplation.
These are things the mass no longer offers, which previously characterized it in both the Low Mass and Missa Cantata / Solemn High Mass forms.
Rather, there is a drabness of experience in the way the mass celebrated in most parishes (it is possible to do a very beautiful Novus Ordo mass, but this is rare, for example, one California Mission near my location does not even bother to use incense at most services, of all things, and is proud of this fact, as though these “bells and smells” are the superstitious holdovers from an obsolete form of Catholic worship that must be consigned to the past in order to challenge the intellectual movements driving the “Secularization” you refer to on their own territory, and also to avoid causing anyone offense, and to dispel potentially negative stereotypes about Roman Catholic worship, when the reality is that this is what so many Roman Catholics are seeking, as is demonstrated by the very high attendance, both at churches which provide the traditional Latin mass, and also at those which seek to conduct the Novus Ordo Mass with great reverence. Also, I would lament that the Divine Office, or Liturgy of the Hours as it is now called, despite the best efforts of the Council of Trent, of Pope Pius X, and of Vatican II, and even the Concilium under Annibale Bugnini, remains mostly the private devotion of the clergy - very few parishes offer a public celebration of Matins (which if I recall is now called the Office of Readings), Lauds, Vespers, Compline or any other offices from the Liturgy of the Hours.
