It is my feeling too, that the high sec war dec system not will improve much before there is something tangible on the line for the aggressing corporation too, giving the defending corporation a way to achieve "victory".
From the top of my head I can come up with a significant personal security rating loss for the aggressing corporation if they fail to cause equal or more damage to the defending corporation by the end of the war. The security loss is applied to any pilot who has been a member of the aggressing corporation at any point during the war. There is already tracking of assets destruction in the current system so this should be easy to implement. It is already the aggressing corporation that majorly decides when to end the war, by stop paying the bill, so they can opt to continue paying the fee and be stuck in the war if they do not want the security loss. In mutual wars this penalty is voided.
From an RP point of view it could be argued that CONCORD dislikes corporations who bribe them into allowing the war but fail to deliver a victory. Bad press and all. Further, by hurting the aggressing corporation on their security rating they will, unless steps are taken to redeem the damage, sooner or later have serious difficulties operating in high sec and play into high sec wars.
Anyway, just a quick idea with room for adjustments.