With the news that screenwriters have voted to authorize a strike as early as November 1, what would happen to your favorite shows were one to occur?
No effect: Sports, Reality, News. They use non-union writers. 6 of last weeks top 20 broadcast shows (and 18-49 shows) are in these categories. Expect the number of these shows to significantly increase if the strike is extended.
Likely no effect: Animated Series. They typically have an entire season completed at once. Unless the strike goes unexpectedly long they will be fine. The Simpsons was the #17 broadcast show for 18-49's last week.
Reruns after a month: Primetime Dramatic Series. Most, reportedly, have about a month or so of new episodes complete. Established series likely somewhat more, new series likely fewer.
Immediate reruns: Late Night Comedy/Fake News shows. With shows written daily, if the screenwriters walk out reruns would likely start immediately for Letterman, Leno, Daily Show, Colbert Report, etc. A long strike might see the shows return with substitute writers or no writers and you'll get to see how funny the hosts are by themselves.
In the long term, an extended strike is likely to drive some viewers to other entertainment more permanently. The writers strike in 1988 may have been one of the accelerating factors in the loss of network viewership.