The problem lies in Pakistan
You are right that the "Afghan mission can still be made a worthwhile one, but not if it is left to drift rudderless". However, it is not the goals that need to be debated but rather the strategy.
Unfortunately, the root of the problem continues to be ignored: namely, the Pakistani military and intelligence's support for the various militant groups in Afghanistan as part of their expansionist policy of "strategic depth". The military and intelligence also play a double game as part of their half-hearted co-operation with Nato forces by attacking militant groups that directly threaten Pakistan's stability but providing early warnings and escape routes during security operations against those that do not. This is done in order to receive financial aid from countries such as the US.
Hence, increasing the number of troops could defeat the militant groups in Afghanistan, but the victory would be short-lived as they could simply retreat into Pakistan as they did in the period 2002-2004 and then infiltrate Afghanistan again, similarly to what happened in 2005 and 2006, as can be observed by the sudden spike in the number of troop deaths from 2005 onwards.
At the same time, the situation would deteriorate if Nato forces were to simply withdraw. The militant groups would be strengthened, and the Pakistani military and intelligence would be emboldened to pursue their policy of "strategic depth".
Given the close co-operation between al-Qa'ida, the Taliban Shura based in Quetta, and Punjab-based groups such as Lashkar-e-Taiba, which was responsible for the Mumbai attacks, the threat to the whole of South Asia would be increased, and in turn to the West itself, since India and Pakistan both have nuclear arsenals.
Thus, we must confront the Pakistani military and intelligence on this issue, rather than assuming that we have, in Obama's words, a relationship of "mutual trust" with Pakistan, and try to make them assume responsibility for the situation. This would allow for a safe withdrawal plan that would also put an end to the Pashtun nationalist insurgency in Afghanistan that has arisen in response to the prolonged presence of Nato forces.