Remember what Baine said. “No life is worth living if we cannot be true to our nature.”
The orcs do live lives of conflict, but rarely war, even on Draenor. A spiritual and honourable, if savage, people. That is their nature.
The Darkspear are similar to the orcs. They live to survive, to please their gods and to respect the spirits. That is their nature.
The tauren are a noble hunter-gatherer people. They have strong hearts and have high morals in general, respecting the land and the Earthmother. That is their nature.
The undead are seen as monsters, but Before the Storm revealed that many are still human on the inside. They just want to live as they did, in peace, even if the world hates them. Nathanos is angry at the Alliance for them seeing his people as monsters, and not leaving them alone, indicating he also just wants to be left in peace. That is their nature.
The blood elves are the ravaged survivors of the Scourge. They live on and show their strength in the magic arts. They are humbled by this, even if they have become darker and bitter, and seek to regain their glory and honour. That is their nature.
The goblins, greedy creatures, live for money. They support conflict if it pays them, but they otherwise keep out of such matters. Many of the goblins we saw on Kezan and the Lost Isles seemed to just be regular folk who wish to make a living, supportive of their friends. That is their nature.
The Pandaren are easily the most peaceful race in the Horde. Hearty and strong, they hold reverence to their allies and aim to bring balance and peace to the world. That is their nature.
The nightborne, survivors of Suramar, have always had a great respect of their ancestors, finding nobility through tradition and holding honour and respect in high regard. That is their nature.
None of these races truly have war at their core, and a desire to seek it out. War always comes to them when an outside force forces them into it, such as the Burning Legion, or their leaders actively declare it. War is not in their nature, and Sylvanas is demanding the Horde not hold true to the nature of the races that make it up.
She is making them live lives not worth living, and that means they are truly dead, truly Forsaken.