Alright so Blizzard themselves never fault the Draenei for not revealing the Legion to the orcs. Glad we got that out of the way, and btw they are the story teller so what they say is canon… thats why we should care imo.
So this question and efficacy of warning the orcs is purely subjective and a matter of opinion rather than facts.
To sum up you think warning the Orcs would have been beneficial.
I would argue otherwise, warning the Orcs would have just made Legions arrival that much faster.
It was a big risk and Velen decided to not gamble on it.
You can disagree but his decision wasn’t grossly negligent. He did the best he could and it wasn’t enough.