I think they'd be crazy to cancel, too, but then I always thought NBC was crazy to put Leno on 5 nights a week.
I am just going by the article I saw and by the actual overnight Nielsen ratings (see below) - what the stations go by as their Bible. Castle is doing better than Leno, but CSI: Miami continues to dominate.
10:00 CBS CSI: Miami 4.0/11 13.83
ABC Castle 2.5/7 10.72
NBC The Jay Leno Show 1.2/3 3.97