This seems to overlook the importance of two other key factors: - International relations, especially that a state nominally supporting progressive ideology was a force that had to be reckoned with - which is not the case today; - the inherent fat-headedness and reactionary sympathies of what HL Mencken called "Boobus Americanus" - and what Richard Hofstadter more accurately attributed to the reactionary nature of the key US institutions - evangelical religion and business - and its grip on the popular imagination, not counterbalanced by progressive institutions, especially organized labor.
Stated differently, the demise of the progressive ideology and causes had little to do with what 'we' (i.e. lefties collectively) did or failed to do, but rather was caused by the oligarchy's renewed support for neo-liberalism thanks to technological advantage it gained in globalization, the demise of the USSR, and the natural responsiveness to right-leaning calls in the US populace. The left did not have a chance against that lineup.
Wojtek