Interesting ideas, but Sylvanas dying does not in fact have almost any bearing on horde infrastructure:that ceased around the time of the wrathgate, where it became clear that her ability to maintain control of her race was lacking. She has no real political connections, Varimathras is dead, the Tauren don't trust the forsaken, the Orcs don't care about the forsaken(and now don't trust them either due to the Wrathgate), the trolls are too busy getting stoned and kicked out of Orgrimmar for loitering, and the Blood Elves only have a diplomatic connection to the forsaken, one that's now largely obsolete. Killing off Sylvanas would actually sever the Forsaken from the horde altogether politically, as the only connection to the Horde they now keep is Sylvanas' tie to the Blood Elves as an undead High Elf.
Jaina on the other hand has significant politically connections. She's a major link to the Kirin Tor for the alliance, the governess of the only neutral alliance settlement, and just about the only character pushing for peace among all the alliance races. Killing her off would cause the alliance to go into a huge political shift towards war.
By comparison, killing Sylvanas off would cause the Horde to shrug and go about their business, as none of them particularly trust or like her anyways, even the Forsaken themselves debatably(who are too caught up in their own personal vendettas to care about anyone else).
My real prediction, however, is for either Tirion, Darion, or both, to die in Icecrown. If Jaina dies I expect it to be in the Halls of Reflection. If she doesn't die there I expect her to be corrupted by Frostmourne post-Lich King or to have nothing significant occur to her at all other than maybe a sob story about Arthas after he dies.