"There must always be a Scarlet Crusade"
If there's one thing Blizzard is known for, it's ticking boxes. When it comes to fantasy D&D, you need all the core tropes present: Human kingdom, Scottish dwarves, haughty elves, barbarous orcs, etc.
And of course, you must include the obligatory "splinter group of good guys who lost everything and are now dedicated to annihilating the enemy, utterly disregarding the pleas for restraint from the Lawful Good Guys who just want to give peace a chance because they're not ALL bad, etc."
The Scarlet Crusade has been wiped out. Completely. Even at it's height, it was being eaten from within by dreadlords, and their evolution to The Scarlet Onslaught and expedition to Northrend couldn't save them. All bases have been purged or literally set on fire. Sally Whitemane, one of the last true leaders, is now one of The Four Horsemen, which "cured" her madness.
But Sally & The Scarlet Crusade was a fan-favorite, with many transmoggers chosing to align themselves with the "Disregard Alliance; purge the Undead" camp. Even back in the old Battle for the Undercity days when the Alliance had a Warrior High King who wanted to wipe out The Horde (if not for Jaina holding him back), you still had many in the Alliance who said ENOUGH, and didn't care that those responsible for the massive loss of life at the Wrathgate were "not representative of the Horde, who are honorable and blah-blah...
So who will take her place?
Introducing: The Army of the Black Moon, or Children of the Ash (Felo'dorei?) or whatever they're called.
It's official Blizzard canon that it was a GENOCIDE (that word was specifically published more than once), meaning the Night Elves lost most of their population just like the Blood Elves did. Of course, the Blood Elves still have their city, while the Night Elves are sleeping on people's couches in Stormwind. But just as the remaining Blood Elves split in half with some staying on Azeroth while others followed Kael'thas to Outland, the Night Elves also split between living in Stormwind and obeying their High King, while others have chosen to follow Tyrande and went off to purge Kalimdor of the orc cancer while their kin sit around and wait for the next inevitable Horde war crime and subsequent quest for identity.
I guess what I'm saying is, The Scarlet Crusade is officially gone from the game (barring one lady at the Darkmoon Faire selling tabards to raise funds), but we now have their one-for-one replacement, because the game world demands the requisite "Disaffected renegade group pushed to extreme measures and will do what the Good Guys won't" trope.
We once had the Human kingdom who are told to live in peace with the Horde because their High King tells them to, while also having The Scarlet Crusade led by the rage-fueled Sally Whitemane who commands her loyalists to ignore the High King and seek bloody vengeance. But now that they're gone, we instead have the Night Elf kingdom who are told to "forgive but never forget" and live in peace with the Horde because their High King tells them to, while also having The Army of the Black Moon led by the rage-fueled Tyrande Whisperwind who commands her loyalists to ignore the High King and seek bloody vengeance.
And the players get to dress up and show support for the renegades, be it transmogging their Scarlet Crusade tabard or going to the Barber shop and darkening their eyes. And we start the clock until Tyrande becomes the end boss in a 5-man dungeon. Not that Blizzard would ever literally repeat history or anything. I'm sure there will be an exciting twist that will totally differentiate the Felo'dorei from the Scarlet Crusade. There's a grand plan. Anytime now.
Okay, maybe that's a bit hyperbolic, but you get the idea. Tyrande has gone from a savage forest elf commander to a haughty Tolkein-esque elf priestess to a "kill 'em all" leader of a band of pissed off elves who suffered one war crime too many ... but not because of any grand story arc by the writers, but because the D&D fantasy world that is WoW mandates a renagade Alliance faction who proclaim they will "do what must be done" and wage bloody war against a not-Tauren Horde race no matter the cost. And with Sally Whitemane and the Scarlet Crusade officially gone, someone had to fill that void.
And Tyrande, a character who had done nothing but be loading screen filler for a decade and gave about seven quests in 15 years got tagged to do the job. Because the writers had absolutely no idea what else to do with her, and Sally Whitemane's "My people were slaughtered, so I am become vengeance" role needed to be filled by someone.
EDIT: With the added bonus of the writers exclaiming "Finally! An excuse to cull the Night Elves!"