What do you mean, 'secrets are not possible'? You can still have secrets, you just stop writing things down, stop talking and literally start whispering or using other anti-eavesdropping techniques.
It’s a thought experiment , as I observe that it is becoming harder and harder to have secrets. Even your examples (whispering, speaking behind a closed wall, even private thoughts) are either no longer safe or have promising technology being actively developed to counter them