The invasion was certainly costly, but not at all futile: It did prevent the Taliban from sheltering Al Qaeda and enabling Al Qaeda to launch many more terrorist attacks against the West. And it did allow Afghanistan to have a much better (albeit corrupt and still far from perfect) Government, giving millions of people, most obviously including women, the chance for a massively better life than they would have otherwise had. Do you really not think that those things are worth while?
Those things are certainly worthwhile, but the cost of building Afghanistan as a nation was to be borne by Afghan, not by British and American - not to mention other NATO members' - lives.
Afghanistan is impossible to govern in any way the West would recognise as legitimate. It was never possible to eliminate the Taliban, only to keep them at arms' length. Nearly all the people in Afghanistan are of the peasant class who, after decades of war, want a cheap and easy life. This makes them loyal only to whichever armed man happens to be in their back garden, or paying them money, or giving them work. They don't really care about the Taliban's aims, or the coalition forces' aims.
The Americans and their allies spent a vast amount of money attempting to train illiterate, stoned, flaky peasants to fight a committed ideology in the Taliban. The Americans had to turn a blind eye to rampant corruption in the local police forces and militias they set up. A recurring theme in local militias was pederasty, where young teenagers were recruited as errand boys but kept on camp overnight for the "enjoyment" of police officers and soldiers.
I'm afraid that the venture of forming a coherent national security force was always folly. For many people in Afghanistan, they will not care too much for the ANA or the Taliban, but if the ANA have been routed out of the village where they've been treating the locals as a cash cow and a source of unlimited sex abuse on tap, I can quite believe that people may tacitly embrace the Taliban and keep their heads down for a quiet life.
Nobody who has been following what's really been going on in Afghanistan over the last ten years could ever have hoped we would leave without the country being taken over by Islamofascists again.
The thing that was futile (and totally unnecessary) was suddenly walking away and leaving so many people to the mercy of the Taliban, in the way we've suddenly done. And I would say it's also likely to prove very costly, in Afghan peoples' lives, in the reputation of the West, in terms of setting back the cause of democracy across the World, in terms of the numbers of terrorist attacks likely to be launched against us in the coming years, and in terms of increasing the power and influence of Russia and China in their attempts to suppress liberal democracy wherever they can.
The exit was a mess. But we had to leave. We do not have a divine right to implement liberal democracy anywhere other than our own country, and setting up a house-of-cards democracy in Afghanistan was a serious mistake.
The fundamental barrier to success in Afghanistan was the sheer delinquency of the people who made up the ANA.
Ever? What if a sovereign state has decided to commit genocide against an ethnic minority? What if they have decided to persecute some group in a way that falls short of genocide but is nevertheless clear persecution? Would you still refuse to intervene?
The question is not about whether "something must be done" but rather the law of unintended consequences.