Sylvanas' actions, while fine for loyalists, were reviled enough to incur a coup. This is more a playerbase issue than an in-game character issue. Theramore probably would have been fine with Jaina destroying Org. But so were the Orcs with Garrosh. And so would the Humans with Varian. And so on with Tyrande and her Night Elves. The characters are all okay with vengeance -- that's the world they're in, the cycle of hatred is real there. But for players, they're divisive. Some are okay with the slaughter, and even ask for things like "bringing the war back in Warcraft." We see these sorts of movements periodically, of the community trying to force more conflict and others sick of the HvA conflict. If Tyrande had gone on a blood craze for too long, it would've pissed one side off too far in the other direction. Too much of one thing. Like Orc oversaturation in WoD.
This way appealed more to a neutral stance -- they got enough vengeance that was reasonable, without making too many people angry about all the blood taken on the other faction. This leaves her enough room to continue hating, and to continue killing later when the flames of war are stoked, without forcing the character to tred Mary Sue powers, or kill-stealing leaders for us, or wasting all those characters' arcs.
The reason isn't to appeal the story. It's to appeal the players behind the story to not go after the character's throat. That's why it's necessary. To create breathing room for the character, to not oversaturate the story with too much Tyrande all at once. She can have a good moment, it can settle, people can be fine with it, and she can get another one later. It doesn't have to all be all at once. Not all major character arcs go on in one full swing that never stops. Because like in WoD, and like when Jaina went on her vengeance streak, sometimes that backfires horribly and people just don't like it. Stopping when the going is good is fair enough. It leaves people wanting more which is better than hating where they're at.
She can still do more, more I'd argue is expected with how Elune is being framed right now with other Night Warriors in the picture, but what matters now is that players at least currently aren't asking for Tyrande's head on a pike. This is an improvement to the situation of Legion where people were hating on the character. It may not have been 100% ideal, but at least it wasn't a disaster following another disaster.