They both have something the other factions wants and expects of their leader. Anduin is calm, passive. Sylvanas is outspoken and headstrong. The Horde misses its outspoken and calm Warchief Thrall while the Alliance expects Anduin to be its headstrong yet passive leader like Varian. Forbidding these traits by allowing each to have a strategic victory [Teldrassil burning and Lordaeron being destroyed] allows the character to continue on the path they are already on instead of evolving and obtaining the other traits their faction needs.
The two both believe in being right and not changing. They've lectured others on how they view them and it has affected them to the point where those same people have died and they were the ones to take their place. The Alliance sees Anduin as too soft and young and Sylvanas as careless and rash. They both mean bad in times where the planet is dying and the fourth war is about to begin. This allows the Old Gods to exploit their weakness and have them indirectly aid them, N'zoth specifically. If the Horde is continuing fighting the Alliance then the focus off of the infiltration and corruption within the two factions is practically ignored. I see the Desolate Council as some sort of Old God infiltration and someone within the Alliance is leading Anduin astray. While everyone is killing eachother, the azerite can be exploited enough to free N'zoth, have him burry his/her way into Azeroth and corrupt her so we can have a way for the Void Lords to finally possess a Titan. This would introduce everyone to the next main primary antagonist and finish up the stories on Azeroth.