It all comes down to trust, doesn't it, in the end, now that prophecy has been shattered.
Trust, or rather the lack of it, is the reason why the gods had to fight Zon-Kuthon to get Him to enter the vault - rather than Zon-Kuthon simply calculating that His interests would be better served by going quietly into imprisonment in exchange for a payment, as He did long ago when Abadar bribed Him to go into an exile that should have lasted longer than it did. Zon-Kuthon in particular, of course, being Void-touched and not something that other gods dare to look at too closely, could not simply have provided Them all with His estimate of how much damage He could do, and go legibly around that, and have that be believable. And so They all had to fight it out, for a day, to learn how much damage Zon-Kuthon could really do, and show Zon-Kuthon how much damage They could really do to Him, before in the end, the vault door could be closed on Him.
Of course, Nethys could have told Them all fairly precisely how much damage Zon-Kuthon would actually be able to do.
A pity, then, that so many gods see Nethys as scarcely any more trustworthy than Zon-Kuthon.
Cayden Cailean actually thinks Nethys is a pretty cool god, personally. This opinion hasn't really moved downwards since Nethys told Cayden Cailean to put an oracle somewhere and then Zon-Kuthon attacked her location and then Zon-Kuthon got sealed and now Nidal is finally being dealt with.
If one is to go around being very logical about such matters, and proving that there's never any point in playing out a conflict, how about if Iomedae updates Her trust in Nethys to match Cayden Cailean's, so that She and Cayden Cailean don't have a persistent disagreement about that?