The thesis I am testing is that the moment when S Pius X started encouraging frequent communion is the moment at which the the mass cultural Catholicism of post-Constantinian Chritianity, in which mass conversions led to a situation in which most people and most societies were not radically 'converted', had been superseded. Some anecdotes from my own unsystematic reading of Irish Church History: a twelfth century Bishop of Ardfert (i.e. Kerry) was reputed to be "chaste". Just think what that implies for the most of the episcopate: they were presumably Caseys, Doyles, and Conrys to a man! I once amused byself by looking at the entries in episcopal registers of the late medieval parochial clergy of Kerry. Time and time again, the record revealed that a cleric was dispensed for illegitimacy. That might mean that most couples were not canonically married and that therefore most children were canonically illegitimate; or, more probably, that these clerics were the sons of priests who naturally planned to inherit their fathers' trade: in either case, it tells us something about society!
But the counter-Reformation implied a clericate different from the medieval priesthood in which a man who could read but had no training could turn up at the Embertide and be ordained (the old system which, like so many of the medieval abuses, survived in the Church of England long after the Catholic Church had moved on). The introduction of seminaries meant a much more professionalised priesthood with an expectation that they would have a more professional attitude to the formation of their laity.
By the time of S Pius X, things were ripe for a new Catholicism in which frequent Confession and frequent Communion could be encouraged. The Dark Ages had finally come to an end. Their ritual marks remained; Communion from the tabernacle rather than within Mass was still common in Oxford Anglo-Catholicism when I was an undergraduate in the 1960s; in both East and West the Body of Christ was not delivered into the hands of the laity; neither was the Chalice delivered into the hands of the laity. Both of these were practices which developed in the 'Dark Ages' out of fear profanation or sacrilege. But, with a more trained and 'sacramentalised' laity, the situation had become ripe for change.
Dv, to be continued.