The Northern Alliance held off the Taliban for close to a decade with token Indian and Russian support dwarfed by Pakistan's active sponsorship of the Taliban with ISI advisers and equipment. A few billion a year in annual US aid would come close to half the entire Pakistani military budget, which is mostly focused on buying equipment to fight India.
I have little doubt the Afghan government, which is dominated by Northern Alliance leaders, would crush the Taliban over time, as long as we're not too squeamish about the methods they use. They weren't bad at fighting - they were just overwhelmed by an opponent funded lavishly (in relative terms) by the Pakistanis, but as of 9/11/2001, they still held a sizable enclave in Afghanistan.
The Taliban are basically Pashtuns, whereas the government are basically non-Pashtuns who were persecuted during the Taliban's reign. And the Pashtuns are around 40% of the population, with Tajiks, Hazaras and Uzbeks each making up smaller percentages. The McCoy vs Hatfield aspect of things will keep the non-Pashtuns fighting the mainly-Pashtun Taliban even without our help. But with our help, they'll prevent the Taliban from coming to power.
Hell no. We need to make damn sure that doesn't happen. They should be paying us.