Since the mid 80's broadcast networks share of the prime-time audience has plunged from 85% to less than 50% as audiences shifted to cable.
Here's a look at the change in average prime-time broadcast TV viewing households during an even longer period. The fall since the early 80's is pronounced.
During the 2007-8 season the top 6 broadcast networks lost on average more than 8% of their viewers from the year before. Adult demo viewer losses were even more substantial, for the CW potentially catastrophic.
I expect overall prime-time broadcast viewership to decline 1-2% in the 2008-9 season. Prior to last year's 8% decline, the annual declines in broadcast prime-time viewership were typically 1-2% beginning in the early 80's. I think that continues. I don't think the industry will recover to its 2006-7 levels (which would produce a gain compared to last season) and then begin to decline.
I think a new "beginning" level was set by last season's results. I think cable's share gains will accelerate as they attack broadcast more directly during their traditional season. I think the shift to digital TV broadcasts in February will be a small contributor to that as well.
However, if declines occur as I predict, expect the industry press to blame them on the disruption in the development process or fall scheduling caused by the strike. I think that's just head in the sand wishful thinking. The continuing shift from broadcast to cable is the elephant in the room that few in the television media want to acknowledge.
Of course the strike by the WGA writers substantially disrupted the season and provided and obvious, easy (and likely substantially correct) reason (and cover for the networks) for much of the audience losses.
Many in the industry (I will link the inevitable stories as they appear) expect the broadcast television audience to bounce back from the strike this season, and, breaking the trends of the last generation, show viewership gains this season. I do not.