The Taliban (on the world stage) is relatively innocuous currently compared to what they could become if they had the backing of China. Of course, aiding an unstable militant group would be risky but, if there's enough money involved, it could happen. (It's not like the CIA hasn't tried the same thing numerous times in the past.)
I think worst case scenario is that we could see a completely Taliban-operated Afghanistan that is heavily backed by China, enough so that China has a controlling interest (so to speak) in Afghanistan's affairs. If Afghanistan were to become a pseudo-vassal state, we could see a US backed Iraq or Israel come into some kind of confrontation with the state, although at that scale we get back to problems presented by nuclear war.
They figure, let the US quell the beast, then go in and make deals with whatever Afghani government is in power at the moment the US leaves. China doesn't nation-build, it makes deals; which makes it a bit of an aloof superpower.