Why do I have to conquer the whole world to win?

I understand all the traditional assumptions, that it is just a game, fantasy with real life elements, makes the game work etc etc. AND the game is called TOTAL WAR.

But it still makes me ask the question: would it not have been better if we rather had a game where it would be MUCH tougher to take and hold a huge empire (as is reality), rather, based on your empire's resources you would have to do other specific missions, such as for Moors to help defend the Egypt against the Crusades? A point based system for such probably would be fun.

In MTW Glorious Achievements worked in a way like that, i would love to have that future cause it was what i used to play after the first 2 months of MTW´s release.