The canon law, as I believe you know, lowers the penalty from excommunication based on necessity, whether objective or subjective (perceived necessity).
If you accept in principle that SSPX had reason to consecrate in 1988, and they believe the necessity to be even greater today (since JPII wasn’t declaring Catholic doctrine “outdated” in encyclicals, or caught on photo at Pagan rituals), then excom is the wrong penalty here.
Also - the supreme law of the Church is the salvation of souls. If SSPX has an indirect impact on keeping canonical TLMs in existence (and the practice of opening FSSP parishes in towns with SSPX chapels seems to suggest this, as does the motu propio Ecclesia Dei), and TLMs/preaching of trad doctrine saves souls, then the doctrine of epikea would seem to weigh against excommunicating SSPX here. Since, like with Card Slypyj’s 1977 consecrations, they are necessary for the survival of a valid, traditional form of Catholicism that saves souls.