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I tend to believe the official severing was in Lumen Gentium when Montini claimed the "Church" subsists in Christ's Church.  

No, that was just bad doctrine.  In fact, many Traditional Catholics (those who believe that non-Catholics can be saved) do in fact believe in a subsistence ecclesiology.

What I'm talking about is it being made explicit.  Before you had JP2 and Ratzinger tripping over themselves trying to "reconcile" V2 with Tradition.  At least they paid lip service to Tradition.  Here Bergoglio explicitly and openly repudiates it.

Here's why I think that the Modernist Heretics finally pulled the trigger.

They were getting increasingly concerned that the Motu crowd were growing in numbers.  Originally it was intended as a fly trap to suck those back into the Conciliar institution who might otherwise be inclined to become Traditional Catholics.  But it was working in the opposite direction.

It was intended to make Traditional Catholics less Traditional.  Instead, it was causing Conciliar Catholics to become more Traditional, and it was also serving as a "gateway" to true Traditional Catholicism.  That's why there's this huge emphasis on not having any more, not having newly-ordained priests offer the Tridentine Mass, etc.  They were worried about the growth of the Motarian movement and its slouching toward Tradition.