Folks, the bottom line is that Jim can't keep Harry happy for long. If he's finally started up a relationship with Murph, then she can't be long for this world.
Good meta-point.
Also...Harry asked Murphy if she might be interested in a romantic relationship, way back in the series, and refused, and her reasons were by no means trivial or silly. The life-span difference (that just keeps cropping up, and for good reason), her own desire for kids and the fact that she's not getting any younger, etc.
Now it's ten years later, or close enough. If Karrin wants to be a mother herself, her window of time is very rapidly closing. Even if she's given up on that, her comments to Harry in
Cold Days remain all too on target. She has a track record of failed relationships, worse than Harry's. Harry doesn't do 'casual' well, as Karrin pointed out, he just about crashed and burned after Susan's departure (though in fairness that would have made most people freak out).
And now probably the 800 pound gorilla, again as Karrin pointed out, is
Molly. Harry keeps saying there's nothing there and not going to be...but at this point I think he's mainly fooling himself. It's becoming pretty obvious to the people that know them that Molly is still in love with Harry, and now she's pushing thirty, she's not a little girl or even a starry-eyed teenager anymore. I don't think anybody but Harry is fooled by Harry's denials that there is something mutual there.
It may never come to anything, of course, but it's there...and Molly has a lifespan, interests, powers, and so on that mesh remarkably well with Harry's. Karrin sees that. She'd be foolish to pretend it isn't there.
I can't shake the sense that the time for a Harry/Karrin romance, if there was going to be one, was probably about ten books ago. Harry was personality-wise better suited to Karrin ten years ago than he is now, the time factor was not quite so bad, Harry's life was more on the same scale of action as Karrin's.
Now, I can't shake the sense of watching a car-crash in slow motion.