I used to hope that the SSPX would be part of the Restoration as well. In fact, +Lefebvre did play a part -- he did consecrate +Williamson.
However, the SSPX is sinking lower and lower, and eventually all the "SSPX good" or fruits will be found OUTSIDE the SSPX, by those who chose God over the World and had to leave earlier or later. Eventually, those inside the SSPX will be all compromised/paralyzed/sold out/useless or even wicked -- exactly like the situation in the Conciliar Church today.
Well, I still think there's hope for the SSPX - a very small hope. And I think that Bp. Williamson has some hope too, but he of course is practical and realistic. What you write above is realistic too. The more the SSPX strives to reconcile with the conciliar church, the more they will compromise, like the FSSP and other former Ecclesia Dei communities. It's not that the FSSP don't have good intentions, but they can't risk being fully traditional when they are beholden to the diocesan authority.
The SSPX only barely resists and confronts the errors of Modernism in the conciliar church. When +ABL was alive, he would point out the errors clearly.
I asked Xavier Sem if he believes that the SSPX will save the Church. I hope he will answer. Because the SSPX can't save anything if it does not confront error. St. Athanasius was exiled many times (mainly by Emperors, I think) because he would not stop pointing out the errors of the Arians. He didn't back down from confronting error like the three remaining SSPX bishops and Fr. Pagliarani have done.
I also recall that Bp. Fellay clearly stated two years ago that he thought that the SSPX would save the Church. That is the same sentiment he held when he wanted to impose the strange theology of that supposed seer Madame Rossinierre/Cornaz on the entire SSPX; that is, until the other three bishops intervened (that was in the 1990's, if I recall correctly). I don't believe that Bp. Fellay has changed his views on the matter.
I mistakenly thought that Bp. Huonder might really just be retiring with the SSPX. Now it's obvious that it was the intention all along that he, a Novus Ordo bishop, would consecrate new bishops for the SSPX.