I can't bring myself to cure her. I see it as a selfish thing to ask of her seeing what she went through to become a Daughter of Coldharbour.
I wouldn't cure her becuase then she'll have finite years.
Pretty much this. We're talking about asking a 3,000+ year old being to sacrifice it's immortality, all for your own selfish wants. A being that could exist for aeons long after the Dovahkiin is nothing more than a pile of gravedust within a decrepit, forgotten crypt. Even the Dovahkiin would be nothing more than "just another chapter" in the long existence of such a being. The Dragonborn is nothing more than a fleeting, passing whimsy, not the focus or even highlight of her existence.
Plus, if you've listened to Seranna's comments about certain areas when you're exploring with her, it's pretty apparent that you're travelling with a being of immense age. Especially when she starts talking about the Dwemer. Seranna has first hand knowledge of an age practically lost to historians, which modern scholars can only guess about. Something so ancient, deserves respect.