I guess the main reason I think she's still alive (at the beginning of a theoretical Book Six) is one of metaplot - if she's dead, that entire part of the world gets a lot less interesting without anything else becoming more interesting. The Red Wedding, although even more disastrous, made Catelyn more interesting as a character. Also, the entirety of the convoluted story that brought Brienne to that meeting will have been an incredible waste of "plot resources" - specifically, pages of the book that GRRM could have excised to bring himself back down to one book instead of splitting into two.

But I could be wrong.