I think the SSPX does many good things. It also does many suspect things, like setting itself up as an authority in annulments.
What it boils down to is this: the consecrations were a grave breach of church discipline.
I understand that the SSPX saw them as necessary and I appreciate their reasons.
The fact is that the leftists wreckers in the Church have done everything with plausible deniability and have shied away from such open and definitive breaches with the Holy See. In the case of Kasper, we have a man who has never formally published or declared his heterodoxy. He has only made suggestive statements and written things that could be interpreted into an orthodox sense. He also carefully cemented his influence in the Church in Germany until he had the majority of the German clergy and episcopate clamoring for his Cardinalate.
I agree with you that Kasper is probably a heretic.
The SSPX did not use such underhanded tactics: they simply said "you told us to do x but we need to do y, so we will do y."
The problem now is that Williamson, again an individual who has never been in a normal canonical relationship with the Holy See, has gone beyond the pragmatic action of Abp. Lefebvre.
Williamson is now demanding that the Holy See acknowledge the consecrations as licit - which they were not as a matter of objective historical fact. That is purely so Williamson can score a debating point and so that he can claim a See for himself as a licitly consecrated bishop.
Williamson's desire to be the boss and his overweening personal ambition have little to do with Abp. Lefebvre's reluctant decision to consecrate him.
Williamson is a problem. I am glad he is in Argentina so that the damage he does is limited. But I disagree on other points. The authority SSPX asserts regarding annulments is a supplied jurisdiction made necessary because of the invalidity of so many annulments granted by various diocesan tribunals. There is a need to sort these matters out before the couples involved can proceed with other marriages.
I disagree the consecrations were grave breaches of discipline. This Pontiff is routinely disobeyed by prelates--in matters of far greater importance. He put his authority on the line to punish the Archbishop precisely because it was Tradition Lefebvre was defending. And therein lies the problem. Does any Pontiff actually have the right to posit an action designed to destroy Catholic Tradition or does he not supercede his own authority when he attempts to do so?
Aquinas and Bellarmine both argue that no Pontiff may command what would harm the Church, insisting that disobedience in the face of such a command would be legitimate. After all, Lefebvre was being put between a rock and a hard place. On the one hand his Catholic faith seemed threatened by revolutionary forces, on the other his fealty to the Pontiff was being tested. In the end, against a background of auto-destructive violence, he sided with Tradition against the Pope. He saw how much devastation was being caused by the progressivists and he remembered the warnings against modernism by preconciliar popes. But it was a difficult decision just the same, especially for a traditionalist.
Yes, the leftists have plausible deniability as you say, but the Pope does not ever press them and avoids confrontation with them. He does not hesitate to confront traditionalists, however--as he did only a few years back with the FSSP when he fired their duly elected Father General along with several priest-theologians of their seminary whom he felt were insufficiently attuned to conciliar thought. This belies the portrait of him as a passive, gentle, saintly man who exists in too rarified a spiritual atmosphere to get down and dirty with subordinates. He gets down and dirty quite a bit with those who espouse Catholic traditionalism.