Orthodox Presbyterians left the PCUSA because practice did not match theology, especially when theology did not change but practice did. Turns out Pius X who opposed modernism was a model for Orthodox Presbyterians (sort of). Boniface explains:
Pius X was not content to simply speak the truth; he put his convictions into practice by taking positive action against Modernism. Pascendi decrees that Modernists be deposed from teaching positions. If they are clerics, their bishops are to place them in the most obscure of offices where they can cause little trouble. Their books are to be censured. The Oath Against Modernism is instituted. Anti-Modernists are promoted while it is made known that no Modernist has any future possibility of promotion (if only that had remained true!). SO vigorous was his assault that the Modernists and progressives complained about his heavy hand.
In short, Pius X never thought merely stating the truth was sufficient; he needed to use the power at his disposal to see it pushed through.
What could conservative bishops do, or have done, that they have not?
Vigorously punish heresy in their own dioceses. Keep strict watch on the activities of certain priests and suspend, dismiss or defrock those who clearly dissent from Church teaching.
Preach the truth boldly, including explicit condemnations of particular groups or ideologies, even condemning heterodox teachers or priests by name when necessary. Go beyond the typical non-offensive, wishy-washy bishop-speak.
Use the resources of a diocese to publish actual informative and instructional materials, not the sort of nonsense most dioceses put out.
Actually issue liturgical directives to promote tradition. The contemporary Church documents offer considerable leeway in how liturgy can be done; the upside of this is that the bishop is given the final call on all of these options. A bishop could easily say, “No guitars and drums at any diocesan Mass”, or mandate sacred chant, or compel every parish to offer at least a monthly Traditional Latin Mass. Novus Ordo Masses must at least incorporate Latin and be said ad orientam.
Dismiss lay persons or members of subversive religious orders from their diocesan committees.
Actually use the tool of excommunication against dissident theologians and dissenting Catholic politicians.
Use resources of the diocese for meaningful ( I stress meaningful) social activism. Example: One priest told me there used to be a scummy motel near his parish that was frequented by prostitutes. He raised some money, bought the motel, and had it torn down. What if the millions raised by our diocesan appeals were used for such uses?
Organize at the regional level and use their weight to push through appointments within the USCCB or elsewhere that were favorable to them while simultaneously using their influence to keep out liberal appointments.
Host guest-speakers friendly to tradition and forbid those who are not.
Forbid Catholic schools and hospitals from engaging in activities harmful to the Catholic faith and actually back up these directives with the appropriate force.
Fire all Catholic school teachers who are in immoral relationships.
Actually celebrate the Traditional Latin Mass and require all seminarians to know it and be comfortable with Latin.
Publicly censure books and films hostile or dangerous to the Catholic faith.
Mandate traditional arrangements in the architecture of sanctuaries and churches; stipulate that no parish has the right to undertake any renovations unless personally approved by him.
Promote priests who cooperate with this agenda and punish those who don’t.
In short, never, never miss an opportunity to promote tradition and actively punish and repress liberalism. Speak the truth boldly but also use the weight of the office to silence, retard, dismiss or dispirit the liberal opposition.
Conservative Protestants who object to contemporary Roman Catholicism are not applying an artificial or alien standard. Protestants who convert to Roman Catholicism were likely never clear about the dangers of modernism or what its chief characteristics were. Modernists didn’t change doctrine. They came along side the world and felt its pain. Real conservatives (Protestant and Roman Catholic) did more than shrug.