I just want you to think about how many racial leaders we've lost so far:
(1) Thrall began being shit when he left the Horde and began his 'spiritual awakening' or whatever it was he was on about, his character became worse and worse until he eventually cheated in Mak'gora and gave his iconic weapon to a random shaman 'champion' that as far as we know could've been alliance (think Varian's sword going to any warrior instead of Anduin)
(2) Cairne Bloodhoof died at the hands of Garrosh who had his weapon poisoned by accident
(3)Garrosh, who's only real mistake (albeit an unforgivable one) was being racist towards the other Horde members, was eventually ousted by the Horde and Alliance then killed by Thrall
(4) Vol'jin got stabbed by some random demon then died
Also, from the comics (Son of the Wolf, released in Legion), we know that Anduin will live to be an old man and that he will be the de-facto leader against the void, whether this is by Horde submission or willing compliance remains to be seen.
My point is that if the Horde loses another faction leader, especially one that has played such an important role in WoW since Warcraft 3, then nothing is sacred anymore. Love her or hate her Sylvanas represents our last bastion of Horde identity as a collective faction under one main leader, otherwise Bliz will be able to put any random npc pleb on the Warcheif merry go-round (who knows might even be the player one day).