As I said before, the War of Thorns did not go as planned due to Sylvanas' ulterior goals.
That is basically what I was talking about, yes. Blunting the Alliance's ability to act, letting the Horde move with relative impunity and slowly bleed it out over time. But again, it's less about making the Alliance "fall apart" and more about preventing it from moving as one. If the Alliance couldn't strike as one without bringing harm to one of its protectorates and thus giving the appearance of either inaction or negligence, it would fracture them. And even if they didn't fracture, they still couldn't bring their full might to bear. Either way, according to Sylvanas and Saurfang's original strategy, the goal was a Horde victory through attrition. Whether weakened in number or weakened in might, a Stormwind handicapped wouldn't stand.
As per A Good War with relevant emphasis:
Holding Teldrassil prevents the Alliance from moving forward unilaterally against the Horde, and they lack the naval power to directly take the fight to Teldrassil with the Horde in consolidated control of Kalimdor. Additionally, Sylvanas believes that if Anduin did indeed plan a strike on Kalimdor despite the Alliance's diminished navies, the Gilneans would be furious at Anduin's inaction in regard to their taken lands - but we know Sylvanas is wrong about that.Originally Posted by Blizzard Entertainment