Y'know I believed the orcs more when they were like "What do you mean that's a Draenei ship?? Heresy!" in BC, unlike in WoD where there's legit just Intact Bits of the ship clearly visible as draenei architecture lmaopic.twitter.com/nEV9lyeSHr
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He was also an outsider and wasn't supported by multiple sources of falsified "proof" like the MU had. But Garrosh's plans in general don't really make sense. And having him go after the Draenei when there were more immediate threats is just weird.
Garrosh is not a sensible person. That was clear as day during his reign was Warchief. He wanted Orcs to rule over Draenor and then conquer Azeroth using advanced technology. And his proof was his knowledge of the future which worked for him.