While taking the city allows us to cut the main transportation link between Raqqa in Syria and Mosul in Iraq, don't expect the Kurds to die in significant numbers to destroy ISIL:
The fighters' apparent escape suggests the Kurds' priority was to seize the town —both for its strategic and symbolic value — not trap and crush the IS forces in it outright. IS's tactical retreat also points to the pragmatism that the group can show when badly outgunned — a contrast to other cases when its jihadis fight to the death, usually during offensives when they aim to wreak as much damage as possible before being killed.
The Kurds are the most effective non-jihadi fighters we have in both Syria and Iraq, but we can't count on them to do much more than defend their territories. So they can't be the force for defeating ISIL.
They can be the force that holds a sanctuary where the forces that can defeat ISIL can be organized and supported, but that's different.
But Iraqis are more willing to die for their country than Kurds are who don't really consider Iraq their country. The Kurds will work with us to fight ISIL to benefit Iraq--for a price--but they won't die in large numbers for Iraq.
On the ISIL retreat issue, I'll at least say in favor of our slow-motion offensive around Ramadi that our initial goal was to isolate ISIL there prior to going after them. That could be a battle of annihilation for the ISIL defenders.
The retreat also demonstrates that ISIL will sometimes retreat rather than mindlessly die in place despite the vows of jihadis that we love life and they love death.
And if we hit them hard enough on the ground with support from the air, they will retreat and scatter at some point rather than die for an obviously lost cause of defending the caliphate.
But if the ISIL forces had chosen to fight to the death in Sinjar, leaving the jihadis there for so long gave them the chance to prepare defenses that would exact a high toll on assaulting forces:
The Kurdish fighters who recently captured this city were impressed to glimpse for themselves a warren of tunnels and underground bomb shelters built by now-vanquished Islamic State occupiers, describing it as a feat of engineering.
The extremist group, which captured the northern Iraqi city of Sinjar in August 2014, has built tunnels in other parts of Syria and Iraq where it administers a self-declared caliphate. But Sinjar is one of the few places where Islamic State has been forced to retreat, offering outsiders a rare view of how the group marries meticulous planning with brutal tactics, such as using the local population as slave labor to fortify conquered territory.
“There are so many tunnels we can’t count them,” Kurdish Yazidi officer Maj. Hussein Khuru Murad said Monday. “This one let them go in and out of a shop on the street, and then make their way to a bomb factory,” he added, pointing to an entrance in the city center.
When you give an enemy time they use it. This time it didn't matter--except to the locals who suffered under ISIL brutality all this time.
But it does matter as a general rule.
So our coalition to defeat ISIL varies in their willingness to die; even jihadis can decide it is not worth it to fight to the death; and giving an enemy time increases the chances that the enemy can create conditions where our side will die in greater numbers.