I disagree, for two reasons.
One, if France expects Austria to roll over Barrois without its help (not an unreasonable expectation), then France has nothing to gain by taking that 25 prestige hit for declining a call to arms, and just break the alliance. If on the other hand Austria was declaring war on a powerful alliance where France's added might had a reasonable chance of making a difference, then it would make a lot more sense to refuse.
Two, the casus belli used does not 100% correlate with the desired peace terms, at least when it comes to players. Fabricating a claim is one of the easiest ways to generate a CB and avoid a no-CB penalty for declaring war, and players do gamey things all the time like declaring war on Barrois when their real target is Barrois' ally who otherwise has other powerful allies they don't want to fight. It would be hamstringing the AI a bit to take a player's declared wargoal as gospel (particularly using a Conquest CB) and make diplomatic decisions based on that.
I have thought about these concerns overnight and I have to disagree with your disagreement.
On your first point, the prestige hit is really only ever a player issue, isn't it? I mean, the AI doesn't deliberately call other countries to war knowing they will decline. Only the player does that. Similarly, only the player declines offensive calls to arms and that's because the game cannot predict the player's behaviour in the same way that it can predict AI behaviour.
On your second point, you seem to be saying that we shouldn't introduce greater transparency into the game by adding a modifier that looks at the declared CB because that CB is often only a gamey device by the player and not what they really mean. I don't think that the fact that the player lies to the game is a reason why the game should not be more transparent with the player. This is about the user experience, particularly for newer or more casual players, and experienced players who are using gamey tactics will find new ways to get round this malus such as creating a CB which won't cause tension between themselves and their ally.
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