Like somebody said, an oak tree is an acorn that held its ground. SSPX had only about 150 priests when the motu proprio was written in 1988. It is now over 400 priests-strong and growing. It has chapels in over 60 countries and numbers over 1,000,000 adherents worldwide. More than its numbers is its influence. It is about one quarter-part of a global traditionalist movement--which is itself the most dynamic aspect of the Church today--but which is studiously ignored by the revolutionary Church. Yet it is the traditionalist movement that is most influential and powerful force on the culture--as exemplified, for instance, by Mel Gibson's film, which came out of the heart of the traditionalist ethos.
And like someone else said, "many will come in my name, saying,'I am the Christ,' and will lead many astray."