Could be getting stale. Could be the judges are not interesting enough. When Stern appears on the other show it will be interesting to see how that works. If he is interesting and ratings increase (beyond Stern's fan boys), that will indicate to me that the audience, outside the core fans of a show, finds the judges to be the actual talent.

So its rating dropped. Big deal. It is still pulling in 15 to 16 million viewers, which is several million more than any other show on television. It is consistently #1 in the ratings, by far (disgusting, to say the least). And compared to most hour long scripted shows, it is extremely cheap to produce. So this is all much ado about nothing.