What-ifs are always tricky, because of so many factors involved.
Maybe if the Night Elves had welcomed the Orcs they'd have become part of the Horde, and all of Kalimdor would have fallen under Horde control pretty much naturally. Maybe that would have created a more stable balance of power so the war with the Alliance wouldn't have happened. Maybe that would have made it much harder for Arthas to invade, and his defeat much faster. And so on and so forth.
Or it could have gone the exact opposite.
Or nothing could have changed substantially.
Who knows.