Yes, it should be worrisome that FN's long-established stars are losing ratings, particularly since they're having trouble launching new shows with new hosts that actually have staying power. Seems to me they're getting into a situation where they have to keep going with some of the long-running shows because what is there waiting in the wings to replace them?
I think, though, that besides the glut of programs featuring the same people, and the fact that some of the new episodes that are getting lower ratings are after hours upon hours of reruns (not everyone is going to bother to see if the 9 p.m. show on a given night is new), there's also the holiday factor and the cord-cutting factor as people cut their cable and move to other (and often cheaper) ways to watch TV. I'm sure there are ways to count those viewers, but they're just not as readily available as the Nielsen ratings.
I would also think that the ratings improve a bit if you add the 50+ demos. Of course 18-49 is the bread and butter for advertisers. But still, the trends are not good for FN.