They care, but they also think it's in mortal hands to stop it from happening. And if it does happen, it should be mortals that fight the demons back and contain or destroy them. They do not believe (correctly or incorrectly, YMMV) that the Marethian Demon problem is sufficiently bad that...
The story pretty clearly bad-ends if you fail. By the time a Crusade arrives, the entire Frost Marches will have been overrun by demons with everyone either dying or losing their souls, including all your friends and potential family up there.
And that's just if you don't buy Kas being a threat...
it fails harder in the Romance route. Kas's starting to get distracted by conflicting feelings and desires and bungles her work.
Not on a scale she needs to get involved with personally, which turns out to be correct -- in the non-Romance route Kas's portal fails pretty quickly and she has to...
-Because Kas doesn't fuck up making the portal.
-Because there's no Wraith.
-Because she lives like a couple miles away and sensed Fuckery on her lawn.
While Ryn has her cage on, she has a special tag applied called Chaste that prevents enemies from using her. She should be usable once it's off, though she'll usually be at the bottom of the priority list.