I'm not convinced that Sylvanas suddenly became a bad participant for being Warchief simply because of two past failures. I mean really, you guys are really saying that two mistakes in her lifetime of centuries of military experience suddenly makes her into not only a bad leader but a bad tactician as well, despite how one of those things inevitably led to the completion of one of the greatest superweapons the world has seen? Talk about being extremely critical about someone else.
It's like if you were in charge of a company and one of your employees gets into an accident that I would come to you and tell you that all of your past accomplishments meant nothing and condemn you for allowing such a thing to happen. I mean really years of your life spent to get to this position and suddenly your life and reputation is completely ruined because of one small accident? I'm sure that Sylvanas would have learned her lesson about trusting untrustworthy people by now. The notion that she would do so again sounds unlikely.
Except their employees aren't Sylvanas.
Sylvanas would have you killed, the raised into undeath so you could work for free! After all, as undead you technically don't have a reason to eat or drink, you're never worried about smelling bad and you can literally work for weeks
without any form of rest. You don't have to worry about medical benefits of any sorts, not even dental, you're dead! You don't even need vacations.
On the serious side, Vynathlon, what Sylvanas has been doing hasn't engendered anyone to trust her in a position like Warchief. She might be a good tactician and strategist, but her actions bad since she became a banshee and they've gone downhill from there. Rampant use of the plague, a -decade- of knowingly using humans, dwarves and other races as test subjects in experiments to create said plague, raising even more dead humans into undeath. Those aren't the actions of someone that's trustworthy. Let alone trustworthy enough top hold the position of Warchief. Gallywax would be a better Warchief than Sylvanas!