Azshara is defeated, but N'zoth is out of his prison. It's more than likely that the Horde and the Alliance will have to team up - i.e. they will need some characters to bridge the gap between the factions and advocate for working together. And Jaina right now is the best Alliance to do that.
Not only is she the only big Alliance character that has along with us been baited by Azshara into helping her release N'zoth and actually saw the Old God go free, but she has also recently successfully worked with Baine, Thrall, Saurfang, Lor'themar and Thalyssra. Out of all the Alliance characters she has the most reasons to go: "hey, let's drop this war BS for now and work with the Horde, because there's an angry tentacle god after us".
I'm not saying it won't happen - sure, Jaina can die fighting Sylvanas, but there's also a possibility that Sylvanas won't get killed at the end of this expansion and that sooner or later she will get some sort of a redemption/justification arc, in which case having Jaina around could be useful.