Nothing persuasive is being offered by Obama or anyone else regards our increasing the security of the United States via upping or even continuing our war in Afghanistan; "just walking away" sounds like one very good idea to me.
We will of course continue to be "threatened" by Muslim extremists, but at least we won't be pretending to be doing something about that via a method that has shown itself without merit in achieving a "victory" against those offering that threat.
We must continue to ask ourselves, since this war isn't really about making the United States more secure, what are our government's goals in fighting it? Many have offered suggestions of what those might be... all of those that seem somewhat believable look pretty awful in any public light, and it is easy to imagine why they would not be offered as reasons for continuing the fight.
What we are currently up to certainly isn't good for the long-term interests of the people in Afghanistan. Either we will eventually leave after a false start of "nation building" or we will be there so long and in such numbers that any reasonable person would have to call it an occupation extended to a "takeover."
Towards what goals?