I think it will be difficult to get people away from the old mentality of war = destruction or all in.
This has been an issue for BM for a very long time and few people trying to change won't work. I am literally reducing a small realm that declared war on mine to a pulp yet they don't want to surrender. Already destroyed most of their capital. I am not even asking much. I just want the gold we spent to get there paid. Not in one go either. People would rather see their realm get destroyed than lose wars.
I am sad to say this but in order to change this, we will need a system where demands need to be forced to a realm like CK2 with a forced peace period of 3 months or so.