If the Horde wins under her, and no longer has the Alliance to function as common enemy and rallying point, what stops them from falling to in-fighting and, potentially, challenging Sylvanas’ right to rule them? With no Alliance, there’s no need for a Horde, just disparate races with wildly varying beliefs that could, very easily, go from being “the group that controls the world under her” to “the enemies she must fight if she wants to keep her power.”
Sylvanas’ entire argument for starting a war at the beginning of BfA was “peace won’t last forever.” As soon as the Alliance isn’t there to unite the Horde, you can bet she’s going to look at “her people” and start calculating which ones need to be picked off because they won’t be obedient to her forever, and she knows it.