I have written about Afghanistan in previous blogposts in April or May this year, so I will not reiterate my arguments. I will only point out something which should be obvious.
The Afghan state could not function (in the sense of possessing a monopoly of violence over the territory to which it lays claim) while the USA/Nato occupied the country ; so it is not surprising that it is failing spectacularly now. The analogy with Vietnam and Laos is often drawn. However, the difference between those situations and that of Afghanistan is that in Southeast Asia, a very disciplined and authoritarian was ready to fill the vacuum left by the collapse of South Vietnam and the previous Laotian regime. (The horror of Cambodia is a completely different story).
There were always only two options, both of them unpalatable:
First, USA/Nato, in order to ensure the stability of the Afghan state, would have to stay there indefinitely as a colonial/occupying power. Biden knows this, and knows that the arguments that 'only one more year' was needed to 'turn the corner' are intellectually bankrupt, and that the ensuing bloodshed (of Afghan civilians) would probably be greater than if they departed.
Second, civil war among Afghans would ensue if the occupying powers were to depart, probably accompanied by the intervention of other powers. As other parts of the world offer convenient bolt-holes for al-Qa'eda and so-called Islamic State, even a Taliban-controlled Afghanistan would be superfluous.
It is not surprising that Biden has chosen the first option ; and even less surprising that, despite its pretensions to 'Global Britain', the UK has decided to follow suit, as it clearly no longer has the capacity (as the recent Defence White Paper tacitly admits) to fight wars on its own.