3 ideas
The novel still holds that the act mattered, because refusing to take part in the lie keeps one person decent inside a corrupt system. On this view, dissent is measured by whether you withheld your own complicity, not by what it achieved.
In the novel's Berlin tenement, the regime rarely needs to watch people directly. Neighbors inform, extort, and denounce each other out of fear, ambition, or self-protection, so ordinary people do the policing themselves. Finders hand in the postcards because merely holding one feels dangerous. Seen this way, a totalitarian state is less a watching apparatus than an atmosphere that makes every relationship a possible betrayal.
Gestapo Inspector Escherich treats the postcard writer as a puzzle to solve with detached professionalism. After he is humiliated and brutalized by his own superiors, and then faces Otto Quangel's calm integrity, he sees that he has served a criminal system. He shoots himself. The enforcer who hunts a conscience ends up exposed as someone who gave up his own.