Dominus Vobiscum: Notes from a massgoer's underground
“Pope Benedict did not “allow” the “old Mass,” and he granted no privilege to celebrate it. In a word, he did not take a disciplinary measure that a successor can retract. What was new and surprising about [his] Summorum Pontificum was that it declares that the celebration of the old Mass does not need any permission. It had never been forbidden because it never could be forbidden.
One could conclude that here we find a fixed, insuperable limit to the authority of a pope. Tradition stands above the pope. The old Mass, rooted deep in the first Christian millennium, is as a matter of principle beyond the pope’s authority to prohibit.”
It’s in the category of what no man can tear asunder.