One of the things that must have been at the back of the Holy Father's, which is only hinted at, in his letter about the lifting of the excommunication of the SSPX, is the problem of rogue priests and worst, rogue bishops.
A real a genuine fear of the Holy Father is sacraments celebrated outside of communion with the Church. In the case of the SSPX, as neither their bishops or priests have jurisdiction, both marriages and the sacrament of penance celebrated by them are as invalid as the baptisms celebrated at St Mary's in South Brisbane, in which the priest used the formula, "In the name of the Creator, Redeemer and Sanctifier". Here is the latest on that story. I hope SSPX priests warn their followers of this before they give these sacraments.
The fact that the Catholic Church considers these sacraments as invalid is an assertion that their followers are still within the bosom of Mother Church and subject to her discipline. We don't, after all, consider Anglican marriages invalid, unless a Catholic is attempting marriage in an Anglican Church.
The situation is pretty horrendous when one considers the effects of a rogue priest, but they pale into insignificance with a rogue bishop, because a bishop can create other bishops. What Fr Kennedy has done in Brisbane will end when his ministry ends but with a rogue bishop the damage can go on forever, they can create priests, but more significantly they can create other bishops with valid orders, thus creating a parallel Church, a schism, the obvious example is the Old Catholic schism from the 19th century, a modern one is that created by Archbishop Malingo.
Schism has always been feared in the Church more than heresy, which is why Popes are always loathe to depose or excommunicate errant, even heretical bishops, better for them to be dealt with gently in the Church, or even quietly grow old and die in communion with it, than be cast into outer darkness where they can do untold damage.
One of the positive acts of all four of the SSPX bishops is that they have not ordained other bishops, apart from Bishop Rangel in Campos, whose diocese was eventually reconciled but have actually sought to have their own excommunications lifted and to be reconciled to Peter, this is truly filial and deserves to be recognised as such. For the Pope, the great fear is that as the bishops grow older, and face death, and no solution is found there is always the prospect that one of them might ordain another bishop and what was schismatic act might indeed become an actual schism.