The problem with trying to periodize all this stuff (i.e. asking 'when did everything go off the rails?', bifurcating time into 'normal time' and 'abnormal' or 'post-normal' time) is that the more one looks into history, the further back one can see the signs. You know someone hasn't looked at history if they think that the rot only started somewhere as late as post-WWII or the 1960s. All I can say is that 'normal' and 'abnormal' are two ends of a spectrum and that the further you look ahead in time, the more all things seem to shift towards abnormality.

Personally, I'd trace it back to Hobbes being to the Left of Filmer, Locke being to the Left of Hobbes, etc. Fast forward to the modern academics (especially those like Adorno, Marcuse, C. Wright Mills, etc.) and they're many of the ones that people can fairly point to as having spearheaded the decline in their respective times and places.