What are the costs and benefits of withdrawing US troops from Afghanistan?

This, of course, is something of a topic of controversy
because we do not know what the costs and benefits would
be.  Some things that might be costs and benefits
include:

The major benefit would be in lives and money that
will no longer be lost.  There is no doubt that the US would have fewer soldiers killed
and less money spent if we withdrew.  A less tangible benefit might be an improvement in
our relations with other countries.  If we were not seen to have a huge presence in a
Muslim country, killing Muslim people, we might be seen in a better
light.

The major possible cost would be the loss of any
control over this area of the world.  If Afghanistan were to once more become a training
ground for terrorists, another terrorist attack might be committed on US soil.  This
possibility is a major cost of withdrawing troops.  One might also argue that a
withdrawal will reduce US prestige because we will be seen to be abandoning our allies
in the area.

Outside of the benefit of less loss of life
and money, these are mere conjecture, though, and arguments can be made against each of
these purported costs and benefits.