The most important cities to the Taliban are Kahndahar and Jahalabad, not Kabul which was left a ruin in the fighting of the mid 90's. There is nothing to fight to keep. So the imminent fall of Kabul wont break the Taliban by itself. It does pose a big threat of a some massive massicares if the NA troops start fighting over it again, or abusing the ethnic Pashtun population. And it gives the US the problem of feeding and carrying for the towns enhabitants. I think that Taliban are hoping that having to take care of Kabul will slow down the US/NA advance.