"Let's swear the oath and form the coalition. We have chosen our side. We must not lose." WRATH said to the others.
It might seem foolish for GREED to give up on the coalition since siding with the coalition will give it the assurance of a reward. But it isn't foolish because it is the only demon God that isn't part of the coalition. That means if the realm tree loses in the era of conquest, it will be the only demon God in the running to become the world-ender. It will certainly become a world-ender even without doing anything to sabotage the realm tree.
The others won't be able to compete with it because of the realm Lord's deal. The realm lord didn't need to offer them that deal for demons to help the realm. Everyone and anyone is allowed to join in to help the realm and the realm lord will reward them if their contributions reach a certain level.
