Honestly, I agree and disagree. The lore of WoW pulls heavily from the Warcraft games, therefore it's really no different than being pulled from a book series. Yes, they can just create whatever lore they want, but that's really no different then saying, "Well they can just write a new book" or "Well the author of said book series is working for the company, therefore they can just write whatever they want and we all just have to accept it as valid."
What people are saying is being misconstrued. They're not saying "There can be no more lore" they are saying "There is no more lore that already exists from which to keep going on." In other words, the current source material is starting to dry up. Any new figures that enter into the fray will be original to WoW lore characters, not ones we've seen, we know, and whose stories have already been told, even if only partially.