Here is an idea. If you don't like this DLC, do not buy it. Vote with your wallet. But we don't even know the scope of this DLC, nor it's price. We also don't know it's release date. Maybe it will be released with the game after early access, for a few dollars. That would be fine in my opinion. It will be an optional part of a single player game and the game is going to be balanced without it.
It will also give modders some new toys to play with. For example, Kingdoms of Arda might include war elephants. Only caveat would be that the players would need to own the DLC to play with that part of the mod. However, this is fairly common practice. Many mods require DLC packs to be owned by the player.