The Neutral forces are the Modrons, Slaadi and Rilmani. Modrons and Slaadi (and the pre-Slaadi precursors) don't get along with each other. The Rilmani do have an army but have to rely on covert actions to keep the balance.I think a lot of neutral forces would side with the celestials in order to maintain the balance, leading to a new blood war between the upper/neutral planes and the lower planes
The idea of there being an Armageddon that the Celestials can't win is also something that many Celestials believe in. It's why a bunch of them covertly do what they can to keep the Blood War going for all eternity.