At least we get to debate, who got Warchief Rouletted the hardest... Vol'jin who did nothing and even his spirit seems to be about to die? Garrosh, who suddenly turned into an orc supremacist and got killstolen by Thrall and tortured in afterlife? or Sylvanas, whose journey of character assassination is still ongoing?
- - - Updated - - -
Well there have been a couple reasons for that working. Malfurion had home turf advantage in Darkshore and he was out of comission, when she was a Raid Boss several years later. For Ragnaros, in Cata it worked, because you were going into the place he was his most powerful to end him once and for all. Generally defeating the Raid Boss before raids usually serves to imply that they are cornered and this is their final stand.
Sylvanas ont he other hand is going on the offensive, with the Jailer to get all the sigils and fulfill their scheme, in which case it's important to build the threat up as insurmountable. There are ways for the story to be written allowing Sylvanas to "win" without diminishing Tyrande, but do we really have the confidence in Blizz to write it that way? I doubt it