That sounds a bit naive. Any hastily organised offensive by the remaining Coalition troops would result only in unnecessary sacrifice of soldiers and civilians. The coalition has pulled out for good, that includes all their firepower. You say it's hard to hold a city w several million ppl. It's a nigh impossible task to take and hold one with just a few thousand soldiers minus overwhelming air, ground support and some commitment. Especialy when only a small part of the population is even siding with you.
All the Taliban would have to do is simply shoot back, dig in and ambush you at every corner, resort back to guerilla tactics, mix with or use civilians as human shields. Fatigue and mental toll will do the rest. To what end ? eventualy get overrun.