I love Mab too but I always felt her character was fated to not survive (even before she had sex with Harry!). But I think Jim likes writing her too, so he won't get rid of her until the BAT, and when he does...I believe there will be some way of given her a sort of happy ending. Like, yes, she does, but she kept a part of her soul after all, and she's going to spend eternity in Heaven...or in Camelot with Merlin, which would be more or less the same.
(By the way, my theory since many years ago is that Mab will ask Harry to kill her. Perhaps because of Nemesis, I don't know).