… the key here is the typical and politically expedient use of “as soon as possible”. Though withdrawal would indeed be the reasonable thing to do, if history is any guide, the UK are not about to do anything at all; certainly not pull out of Afghanistan. The West will eventually pull out of Afghanistan once the situation there becomes militarily untenable and the number of casualties amongst Western troops becomes politically untenable at home. Till then, the UK, the EU and the USA will blunder on and, despite occasional attempts at soft intervention, Western strategy will become gradually more lethal, more destructive but more detached thus less accurate inflicting devastating losses on the civilian population and the environment.
The thing is that with everything that is happening at home, the West can ill afford greater numbers of casualties that are necessary if the country is to be “pacified” because pacification can only come about with boots on the ground. And considering Afghanistan’s size and topography, nothing short of a good million troops might do. In the absence of boots on the ground, the West must rely on air power and missiles… hence the tolerance for collateral damage must necessarily increase.
Of course, in the event of a global conflict and in the event the conflict should start or be extended to that part of the world, then increasing boots on the ground won’t really be a problem. But till then, the West will stay the course and only pack it in once the current puppet regime is discredited, overthrown and/or casualties amongst Western troops create a political backlash at home.