The upfront advertising sales season is upon us and this year will be the most critical year of all – setting the scene for the inevitable collapse in upfronts that will be triggered by the growing proliferation of programmatic advertising, together with the snowballing fall from grace of TV ad ratings. Broadcast and cable networks saw an unexpected upswing last year, with ad sales increasing $800 million to $18.6 billion in the completion of summer 2016, but they fell the year prior to that. Although ad sales may continue to rise this year, in the next couple of years or so there will be another slump on a much more calamitous scale. The only thing really preventing a mass-migration to…