Yea, that idea was rejected. It was considered to avoid TMP, but it was said it would not be acceptable. Something about war being war and such.
You're missing the point. This isn't a friendly, we're totally not going to do anything permanent war. This is a real war. You keep whatever you take (and while I think it's unlikely, if you completely wipe out the other realm, then they stay wiped), and the stakes are quite high since your realm rank would be determined by your victory or loss, and the Ruler of the whole thing comes from the highest ranked realm. Especially since there is no TMP, this is not an attempt to avoid game mechanics, this is an attempt to provide a succession for a multi-realm entity in a way that makes sense and is fun.