I think you missed a little of the nuance of the traditionalist position. Pope Pius X did everything possible to eliminate the modernists. And he was successfull at doing so, as successful as is humanly possible. But despite his best efforts, there were still some who "went underground." They were condemned whenever they popped up with unorthodox books and so forth as late as the 1950's. A great many of the periti at Vatican II had their works condemned just a few years before by Pope Pius XII.
It's just like getting rid of communists in the US government. If you do nothing, or even encourage them, they way that Roosevelt did, then your entire administration will be run by communists and you will end up handing over all of Eastern Europe to the Soviet Union. But even if you make your best efforts to eliminate every communist from the government, as was done during the McCarthy era, there will still be a few who escape detection who will then pop back up when the coast is clear and a new administration gives them freedom of action.
James Jesus Angleton was called "paranoid" because of his "unceasing vigilance" to uncover communist spies. People said that he was proven wrong when he didn't find any. But in fact that was the best proof of his success. After he was eliminated, there were plenty of spies to find.
Same thing with modernists in the church. People claimed that the Vatican was paranoid in the earlier decades of the 20th century when their "unceasing vigilance" turned up few heretics. But once the new administration of John XXIII sent out the all-clear signal, then one could find heretics everywhere.