Back in January a made a thread asking if the Alliance would forgive the Horde . Most of the responses to the question were against forgiving the Horde; often in favor of killing, dismantling, imprisoning, or putting us in some sort of probation depending on our race and involvement in the war. There were others who weren’t even in favor of forgiving the Horde back in MoP after the Garrosh debacle. Certainly there are in-lore Alliance characters who share the same sentiment.
What does this mean? It means that if you side with Saurfang, you’re at best really pushing your luck with the Alliance forgiving the Horde a second time, and at worst you’re inadvertently aiding the extermination of your own faction. You may not like Sylvanas or her methods, but keeping the Horde together is better than inciting a civil war in the middle of an actual war. There is no doubt that even if Saurfang’s rebellion succeeds, we’ll stand zero chance against the Alliance forces in the aftermath. We’ll be too weak from in-fighting. The Alliance can simply do whatever they want with the scraps left of the Horde. Saurfang may very well be the Alliance’s best (unaware) ally in this war.