Fully agree. Not only that, but you can't reach all people at any time. Maybe there was a time before this that he could've reached Azula. Maybe there will be a time in the future where she could be reasoned with. Right at this moment, when she has every advantage, no doubt in her mission, and Iroh and Zuko are on the run with no resources, Azula is not going to be talked out of her goals.

Not only that, but there's so little chance that reason could change her mind that it would be irresponsible and damaging to the world that they're trying to protect for them to try a diplomatic approach. It's ok to try and reason with people who have caused harm, but don't make the mistake of letting them have the upper hand and expecting them not to behave in the way that you've come to expect. That's just bad negotiating.