Well...I did post the article regarding the plummeting ratings this season (the winter hiatus episode being the lowest rated episode ever) . I haven't checked the ratings for Monday's episode yet. I have a suspicion that "Castle" could end either this season, or perhaps be granted one last season next year. So far though I've not really had a problem with this season. Monday's episode was clearly a filler episode. I'm still convinced that the second half of the season will refocus on the JB murder case, we will be revisiting the Senator as well, and possibly be introduced to Rick's dad finally.
My gut tells me that declining ratings this season is due to Castle and Beckett finally getting together. Since the show began, we have been teased with those two characters getting closer. There were a lot of fans invested in "Caskett" and I wouldn't be surprised if they got their payoff in the final episode of last season.
All that would await those fans this season is watching what kind of obstacles the writers would put in the way of the happy couple. Those invested in the relationship may not have stayed to see that.
So while the show may not have creatively collapsed because the leads got together (ala Moonlighting
), the relationship's fans may have decided to leave before it happened.