Like I said, if I'm a fan of any person when it comes to Castle, it would be the writers -- I'm a huge admirer of the work of Old Man Winter and Dara Creasey. It's a character-driven show, though the chemistry between Fillion and Katic was definitely more important in the early years, because the whole "muse" thing had to be believable.
The fact is, Fillion is bankable, and Katic presently is not, no matter how much the Stanatics like to deny it. Every film she has been in has been a box-office bomb (except for her bit part in Quantum of Solace, though for a Bond film, that one was a low-earner), whereas Fillion turns just about everything he touches to gold. Even Firefly is now a profitable product due to DVD sales, syndication and merchandise. Part of the problem that Katic has is that, out of character, she is not a good interview -- she either comes across as an airhead, or as a disinterested intellectual (which I know is closer to the truth, but that breathy voice she has IRL works against her.) Fillion is the goodlooking guy next door who likes to tell jokes on himself; people who don't know his work see him in an interview and imagine he could be their best bud.
Speaking of best buds, another interesting thing about those two is their approach to publicity. Fillion is a master at it, making himself VERY available but on his own terms. He fools fans into thinking that they know him very very well, but actually reveals very little of his personal life. Katic, OTOH, is one of the most secretive actors working today; she reveals essentially nothing of her own life, and guards her off-screen privacy as if she were an covert agent for the CIA. While that approach may work for someone who is as stunningly good an actor as Kevin Spacey, for example, for most actors trying to build bankability it isn't an effective strategy.