By no means am I against objections. Again, Francis and Dominic had objections. But, as you say, the “form” they used was proper. Please note as well that Francis and Dominic followed proper canonical protocol and obeyed the authority of the Pope.
Regarding the excommunication(s), there’s a few fronts to look at. One, the necessity portion. As I may have mentioned elsewhere, the Pope said there was no necessity. Bishop LeFebvre thought that there would be no further valid priests and no further valid Masses, and that Sacred Tradition would be destroyed. The Pope (and the Popes after) specifically said that those were not true; there would still be valid priests, there would still be valid Masses, and Sacred Tradition would remain. And of course, there’s no necessity clause for the Sacrament of Holy Orders, nor any for consecrating Bishops.
The part detailing being “bound in conscience” is pretty rich, considering how often those who support the SSPX have issues with others following their conscience. But sure, lets roll with that. The SSPX website that Bishop LeFebvre had to perform “a subjective mortal sin” to be valid, and they say that because he was “bound by conscience”, he didn’t sin. The primary issue here isn’t just that the same justification can be (and probably is) used by liberals for their own acts (although that’s certainly a big issue), the bigger thing is that the sin of disobedience by a Bishop towards the Pope alongside with the direct violation of Canon Law is likely an objective mortal sin. This isn’t a case of habitual sin or invincible ignorance or living in a certain time period or growing up in a harsh environment. This is a man who was a Bishop and knew exactly what he was doing, received multiple warnings by the one who had the authority to do so, and went ahead and did it anyway.
To summarize, what this boils down to is two things. The first is obedience. Within the consecrated religious, you are not bound by your solemn vow of obedience if your Superior will cause you to sin. In this case, the Pope promulgated a valid Missal, the Sacraments remained the Sacraments, and Canon Law was validly revised. There was plenty of change as it pertained to customs and practices, however those are subject to change by the one who has valid authority. There was no break from doctrine, zero. How doctrine was communicated was changed, but no deviations were present.
The second part is pretty simple. It’s the office of Kephas and who holds the Keys. Everything in the situation with the SSPX was done with the valid successor of Kephas exercising the office of Kephas… Canon Law rulings? He’s the judge. Suspension of priests? He’s the judge. Faculties removed? He’s the judge. None of these were abuses of power, these were valid actions by the one who has the final ruling when it comes to Canon Law.