IMO, the MSM started down the path toward being unpaid partisan prostitutes in the 1960s, and people started, slowly and gradually, waking up to their game some time during Carter's MalAdministration. The MSM kept going down that partisan path, increasingly with every election, with boosts with every R President. And bursts of recognition with the advents of ConservaTalk radio and the Internet - more people waking up to their game.
What makes 2015 and 2016 stand out, IMO, is that the MSM tried to torpedo a candidate during the R Primaries, and failed, abjectly. And then failed to take him, Trump, down during the general election campaign.
If by some magic from an alternate universe the MSM admitted what they have been doing for 5 decades, apologized, and became non-partisan, it would take decades for the MSM to come close to the cautious trust they enjoyed in the 50s and 60s.