the fact is, romantically, Beckett is clearly replaceable for him

I can see why you would say that. Although, again just personal opinion here, I'm not sure I'd use the word replaceable. It's part of a whole sex vs intimacy concept that I've got going for Castle. And my reasoning, in part, is due to this season's premier, so I'm just going to stop there since I don't know that you've seen it, yet. :D