I actually had a pious lady ask me this (in person, after a Resistance-affiliated Mass). She thought it should somehow bother me, or make me pause a little in my opposition of the neo-SSPX.
Because, after all, the following things are true and not disputed by me: the SSPX was a Providential work of God, it has done countless good over the years, helped to convert and save many souls, form countless Traditional priests, including many good ones, and its founder (+ABL) was wise and saintly. His mother was also a virtual saint.
But the answer is simple:
Just because St. Ignatius Loyola was a saint doesn't mean I shouldn't oppose the modern-day Jesuits. And all the other religious orders founded by canonized saints (Franciscans, Dominicans, etc.) have fallen prey to Modernism and the Conciliar Church. But their founders were rock-solid in the Faith!
This is actually nothing new. So no, it doesn't give me pause at all.
In conclusion: Our Lord never made a promise that his organization (SSPX) would endure or stand fast until the end of time, or any such promise. Our Lord did make a promise to St. Peter, but +ABL never received such a favor.
God is not a respecter of persons. With God, it's not "who you are" & "who you know" but rather "what you do". If you want God's blessing, then fear the Lord, follow His law, and do His will. Just read Scripture back to back and you will see this. All that matters is being faithful to Him or not.
If God allowed the Conciliar Church to become the steaming mess it is today, He will certainly throw out the SSPX like salt that has lost its savor, whenever that action is called for. He has not bound Himself by any promise to the contrary.