"These effects must be due in part to something else. Protestants are also the subject of jeers. "
Just the opposite. These effects are due to the exact same cause--modernism. The New Theology was a virus that passed from the mainline churches to Catholicism by way of the Lutherans in Germany. The German bishops led the charge in Vatican II. Lutherans participated in the fabrication of the Novus Ordo.
Look at which churches have been losing steam since the sixties: the liberal ones. The mainline Protestant pews are empty, not the pews of evangelicals. Nor are the pews of conservative Catholic churches empty. It is liberalism, catering to the world, that destroys the faith, nothing else.
When I was in the seminary back in the 80s, there was much visiting back-and-forth between seminarians of mainline churches and ours. They were subject to the very same deconstruction of Scripture as we. They had their Raymond Browns and their Walter Kaspers. So it wasn't "something else" as you say. The cause for collapse was the same--accommodation to modernity at the expense of faith.
***Just the opposite. These effects are due to the exact same cause--modernism.***
Though I believe that is a definite factor I wonder if it might not also be attributable to something else - sin and compromise in the life of the real believers.
We know the Church is composed of wheat and tares, the saved and the lost. Churches which preach modernism have a strong appeal for who don't really want to know the truth. Specifically the truth about themselves and their sin before a holy God.
Modernism is mainly composed of tares (no doubt with a few confused stalks of wheat thrown in for good measure!) As such they are "non-players" in the true game - the invisible game on the spiritual plane. They are not really on God's team.
Those people who truly believe, who have accepted the message and the rule of Jesus in their lives no matter what the personal cost - these are the ones who are spiritually alive and they are on God's "team" so to speak.
Now the question is why is God's "team" loosing? Could it be because those who truly belong to God are not living for Him as they know they should? Could it be also that they are spiritually "alseep" having been seduced by the cares and riches of the Western world and have allowed sin and compromise to go undealt with in their lives?
When is the last time you heard a priest or minister strongly rebuke sin in the lives of his congregation?
A side note. God's "team" seems to be winning in the third world (Ex. China has seen the growth of the underground church from appx 900,000 to 100,000,000 in a one hunderd year period - Africa has a similar growth rate.)