I understand the concept very well. I just also see how this would actually impact gameplay in practice instead of just in concept. Maybe read your own thread.
I don't think you know how wars in V3 work, because your example breaks the rules of the game.
To go to your example Britain + Portugal are fighting Spain + France
Britain has wargoals on Spanish territory
Britain and Portugal beat Spain and sign a separate peace. Spain is not signing a peace with Britain, but in fact the entire alliance. This is how the system works as described in the dev diaries. Participants only leave the war when they are defeated. If they have achieved their wargoals in a separate peace, they still remain in the war.
Britain remains in the war on Portugal's side and continues to fight France.
Now under the proposed system, France would receive wargoals to liberate the recently conquered Spanish territory.
France can only push those wargoals as part of peace negotiations with the Britain + Portugal alliance, or in separate peace negotiations with Britain alone. This is how peace deals have been described to work.
If somehow Britain exited the war, and only France and Portugal were fighting, then France could demand that Britain give up territory while it was making peace with solely Portugal.