It started sooner than this. (I have not watched the documentary yet, response to comments). Source - I am 56 years old and read newspapers every day.

In the 1970’s Gannet bought up local papers, filled them with associated press news wire articles, and laid off the local reporters. The major papers (ie New York Times) followed a little later, fired most of the “reporters” to replace them with A/P wire stories and corporate funded media releases.

From “Woodward and Bernstein” took down a president it took five years for 75% of investigative reporters to be laid off.