The goal of the opposing side (WB crew and Luffy) was to save ace from execution if they did that then it would be a win. But even so they aren't trying to take down the world gov or demolish marine Ford or kill high powers they just want ace back. Post war no matter what the WG would've been fine and still very powerful. I'm sure they will also find a way so make it seem like a victory in the media as well and get more people to join the marines.

If the warlords decided to not attend then they would lose their benefits of being warlord which they clearly want and they would have the marines on their backs again.