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You could argue that that Arian heresy is an example of a prolonged R&R situation (let's not get into the weeds on the "kind" of R&R, just a general point).  You had a weak, ineffective, heretical-condoning pope, you had (according to historical accounts) 98-99% of the catholic clerics/population infected with the Arian heresy, you had self-espoused "arian-catholic priests" arguing with "I-agree-with-Arianism-but-not-your-kind" priests, and then you had "St Athanasius against the world", the only (maybe a handful of others) cleric who was truly orthodox.
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After Arianism went away (remember, Arianism had already been condemned multiple times before the time of St Athanasius, just like the tenets of Modernism have been condemned previously), there was not a re-consecration of bishops, nor a re-election of Cardinals, nor a re-election of popes, nor a re-installation of bishops into dioceses...the heresy went away and orthodoxy return when a 100% orthodox pope was elected (who also censured Pope Honorius).  You can easily argue that 98% of the clergy at the time (including the pope) led people into error because as it was said "the entire world groaned under the error of Arianism."  Very similar circuмstances to V2 and indefectibility didn't apply.

Neither do R&R Trads today, because there is nothing in V2 that is binding on anyone’s conscience.  And the new mass is not obligatory.  +Vigano’s view of new-Rome agrees.
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The period of Arianism is even MORE similar to today when you view that Honorius’ lack of condemning error (while never officially condoning it) created a chaos vacuum where the catholic faithful/clergy didn’t have a clear authority and guide.  So what happened? This left the Arian clergy free reign to assert authority and declare Arianism to be true (I’m sure they used Honorius’ words against him), just like the Modernists bishops used V2’s ambiguity against the faithful and also Paul VI’s contradictory statements on everything.  In the end, both then and now, the evil bishops/priests were the ones that corrupted the people.  The popes created confusion by lack of leadership (Honorius) or by a contradiction (V2 popes).  But OFFICIALLY neither Honorius nor the V2 popes have ever commanded sin or error to be accepted.

Is that the main gripe then?  Was secularization commanded under pain of sin?  If not, that has nothing to do with the magisterium but with political policy and the Vatican govt.  You can be in a secularized country and still save your soul.  Is it ideal?  No, but secularization was already happening to all catholic countries since the 1600s, due to Protestantism and Fɾҽҽmαsσɳɾყ.

Consequently, in order that relationships of peace and harmony be established and maintained within the whole of mankind, it is necessary that religious freedom be everywhere provided with an effective constitutional guarantee and that respect be shown for the high duty and right of man freely to lead his religious life in society.

May the God and Father of all grant that the human family, through careful observance of the principle of religious freedom in society, may be brought by the grace of Christ and the power of the Holy Spirit to the sublime and unending and “glorious freedom of the sons of God” (Rom. 8:21).

Each and all these matters which are set forth in this Declaration have been favorably voted on by the Fathers of the Council. And We, by the apostolic authority given Us by Christ and in union with the Fathers, approve, decree and establish them in the Holy Spirit and command that they be promulgated for the glory of God.

I, Paul, Bishop of the Catholic Church

There follow the signatures of the Fathers.