What we think of as retreat in the modern day just couldn't happen with Medieval levies.
You imagine some... highly trained and motivated army, when really what you had was an armed gang of people who really, really didn't want to be there. Once they fled from combat, they were not running from the battle they were running from the war. An "orderly retreat" with such a force will very quickly collapse into a total route even with trained and disciplined troops. With a levy, it's almost a guarantee.
No sane general would all but ensure his army's destruction - remember, something like 90% of casualties from enemy action happen after the battle and during the route. Point of fact, fighting to the "crushing defeat" you describe increases the odds of the enemy routing and getting massacred. Running in this day and age meant getting chopped to pieces quite brutally: in order to run, you must turn your undefended back to the men carrying pointy sticks.
That's my issue with Shattered Retreat, it's not even close to what would happen to a levy based army system. Units that found themselves shattered generally were eliminated as a fighting force, even if all the men didn't actually die. Not since the Roman legions, and not until Cromwell's New Model Army were shattered units able to reform and come back to fight again. In fact, that is what made the New Model Army so powerful at the time, the unit cohesion and ability to reform after charges and pursuits were extremely atypical of the time.
- 9