she is just pretending to be weak.You do realize if she made a Scourge type plague, the Argent Crusade would mobilize and join the Alliance in wiping her off the face of Azeroth right? This is the epitome of being a fanboy. It's been stated in the lore already that Sylvanas can't defend her land and is fearful of invasion. No hypotheticals. Just canon lore.
She simply doesn't have the manpower to hold up in a long battle and if she were to be the aggressor on more than Gilneas, she has a lot of groups who want her head on a stick. She's already being eyed by the Ebon Blade and Argent Crusade as is.