I don't think an SSPX-FSSP merger is likely. First obstacle is Pope Francis. The things he says get odder and odder. I used to think him a cunning Modernist, but while he was politician enough to become Conciliar Pope, his monologues are of a man with failing mental processes. The second obstacle is related in that people who see a hasty reconciliation with Conciliar Rome and then to a priestly society long loyal to Conciliar Rome, will just exit. They might go to SSPX-MC or SSPV (not sure if they are Sede) or another priestly society, perhaps even a Sede society. The third obstacle is there are two priestly societies who have in the time of their existence taken their own direction, even if they have a common ancestry.
I just think it unlikely, but Matthew has his sources. We will see.