Just a short comment on realism:
While it may be more "realistic" that the archers would retreat as soon as the infantry line breaks, it would also be equally realistic, that even after retreating around 50% of them would be run down and killed, and even more if the opponents have a good contingent of cavalry. Thus, they would likely die while running without causing any damage. As was stated earlier, massive damage was done to armies on the retreat.
Quite simply the retreats would have to be much more devastating to keep things in line. Neither change is beneficial towards the game balance or fun.