The point is, you don’t need human investigators to find leads, you can have the algorithms do it. They will find people of interest, assemble the watch lists, and flag whomever you like for further tracking. And since the number of actual terrorists is very, very, very small, the output of these algorithms will consist overwhelmingly of false positives.
The security state operates as a ratchet. Once you click in a new level of surveillance or intrusiveness, it becomes the new baseline. What was unthinkable yesterday becomes permissible in exceptional cases today, and routine tomorrow. The people who run the American security apparatus are in the overwhelming majority diligent people with a deep concern for civil liberties. But their job is to find creative ways to collect information. And they work within an institution that, because of its secrecy, is fundamentally inimical to democracy and to a free society.