The battles are a bit small scale and resting only on a single commander in TW games so making units break easier doesn't work very well. If there could be multiple commanders and the battles were large enough that a break in the line actually took some time to exploit before the entire enemy routed then maybe.

As it works now though I think its not so different from what you are saying except for the habit of the AI to kill off its general and thus lead to much faster rout than normal. To rout an army quickly you still have to flank or separate some of their units from the command radius of the lead general either by killing that general or drawing off either the general or the units. The main difference is the number of men who die in the actual battles compared to historical battles. However in history after a battle the fleeing side wouldn't normally reform and attack a few weeks later. There are also examples of the routing side being nearly completely wiped out but that doesn't seem the norm. I'm fine with the current system since the engine is rather limited. There also isn't sickness or disease or attrition when campaigning long which definitely did kill many so overall it might not be so far off. IE- a long campaign in the game with 3-4 battles fought might see an army lose 50% or more just to the battles whereas in history a victorious army might lose 5% casualties if the enemy broke so that would be only 20% losses- however in the game no one dies from bad water or infections from a minor wound weeks after the battle.