The Horde has bit hit hard yeah. But this isn't exactly the first time the Horde's been hit. No matter how many times the Horde takes a dive we rise back up and learn from the suffering we endured.
Liiiiiiike a Phoenix~
Garrosh was a lesson in Leadership. Battle for Undercity was a lesson of Obersavation. Warlords of Draenor was a lesson of don't let Garrosh time travel.
The Horde is slowly growing back in strength after the events of Siege and despite the losses from the Broken Shore (which was honestly both sides) and the coming Battle for Lordaeron we'll still endure. Time and time again the Horde has proven to be equal and even at times stronger than the Alliance's. Vice versa with the Alliance.