The first private conversation that Martin Luther King, Jr. had about the injustice of his time was captured and automatically analyzed, and as a result he was identified as a disturber of the peace and arrested for unrelated violations long before his message ever reached an audience.
Rosa Parks was detained for disorderly conduct long before she made her stand, and as a result her stand never happened.
There was never a zeitgeist of change in the oppressed community, because there were never any stories of people resisting. They were all prevented from acting in any significant way, or from getting together to share their stories and find support.
The issue of racial discrimination never made it to court because all of the people who could have brought it there were stopped before they could get started.
Perfect surveillance can stop any threat to the status quo while it's in its infancy, long before it amounts to anything. It can probably do it without introducing any new, draconian laws by simply looking through recent history for "questionable" actions and harassing people over them.
The argument that it's okay now basically amounts to saying that there is no longer any injustice and that today's system is a perfect example of virtue and fair play, and that anyone who disagrees is a malcontent who really does deserve to be silenced.
Sometimes the majority is wrong, and it takes brave people willing to break the rules in order to point it out and change things. I'd say that democracy does depend on privacy for this reason.