Right, but Scotland and Wales are already Democracies.
A similar scenario would be Scotland or Wales being run by a Theocracy or on a feudal basis, and the farms being profitable and the farmers content, then along comes the US and installs a puppet government in London which subsequently gerrymanders the elections, the farms are rendered inviable, and the farmers and their families starve due to a lack of any alternative means of earning a crust. Do you think the Scottish and Welsh farmers are going to be big fans of democracy under those circumstances?
It's not as if people in the UK are above reacting to governments and entities who threaten their livelihoods. There are plenty of historical rebellions and riots to point to, and as little as 40 years ago pitched battles between miners and the police.
The jist is, the democratisation of Afghanistan was never going to be a success because Afghanistan simply wasn't ready to be democratised, but more than that, the US never tackled the fundamental problem of 85% of the country still being entirely beyond the control of centralised government. The apparent "liberty" in major cities was never anything other than an illusion, because it was only that way due to massive US logistical support, and the fact the Taliban couldn't forcibly remove the US and foreign militaries from their encampments.
Once the foreign militaries left, there was absolutely nothing to prevent the Taliban rolling in and restoring the situation pre-2002. As already covered, the Afghan Military was nothing but a notional "on paper" entity that didn't actually exist as a legitimate fighting force, and even if it had, it still stood no chance at all due to beginning from a position which was already futile. The US maintained it's military presence only courtesy of it's ability to supply itself by air. How is the Afghan military going to supply itself? Where are they going to go when they have to cede territory due to a lack of cohesion, no command structure, and no logistical support? They can't just retreat to the next nearest town, because they are surrounded on all sides due to the fact the US never bothered to secure any territory beyond the limits of the towns, cities, and bases they could defend with relative ease.
The US's one real success in terms of forcible Democratisation is Japan, but that involved 7 years of occupation and military rule, a further 15 years or so of "soft" occupation, immersing Japan almost entirely in US culture to the point whereby Democracy was almost taken for granted as the norm, and Japan had, for decades prior, been desperate to industrialise and modernise itself in any case, so it was absolutely ripe for development into the nation which emerged in the 1960s. Afghanistan is in no way similar, and neither was the US effort in that State over the past 20 years. Personally, I think that it was always apparent to the US they had got themselves involved in a futile proposition and it was just a case of who had the gumption to rip the sticking plaster off.