I used to think that because the SSPX told me so and then, when I jumped into the Resistance, I was outcast for that as well. Then I decided to read what the sede position is from actual sedes. If the pope is the pope, you can rest assured his Mass is legit and he's enforced it onto the entire Church.
THIS^^^ ... there's even an anathema in Trent that strongly implies this.
You'll notice that SSPX never use what's really THE strongest and, in reality, THE ONLY argument they have in favor of the validity of the Conciliar Holy Orders, namely, that a legitimate Vicar of Christ simply cannot promulgate a Rite of Mass that is invalid. Problem for them is that this same principle would prevent a Vicar of Christ from promulgating a Mass that's harmful to souls and displeasing to God. It's absolutely impossible and entirely inconsistent with the Promises of Christ, that the Catholic Church under the legitimate hierarchy could possible use a form of Public Worship that displeases God. I'm simply dumbfounded by those who can make this claim. This would constitute an obvious defection of the Church from her primary mission.
While Archbishop Lefebvre himself made the necessary distinctions, unfortunately they were subtle enough and not sufficiently emphasized, so that some of his legacy has become people who have lost faith in the Papacy and the very nature of the Church, where they consider it to be similar to some political institution that can go corrupt, once in a while going "Left" and then under new leadership going back to the "Right".