If the Taliban seriously threaten to topple the government, or if the government looks like it's about to collapse and run. This would not be a good outcome for us, nor anyone else.
The US might have withdrawn, but they've not entirely disengaged. Neither they, nor Iran, nor Russia, nor even Pakistan want a Taliban unitary government to seize power. Every single player prefers a more balanced settlement of one sort or another.
The US still could engage the Taliban using their carrier strike groups. Even now they have three carrier strike groups converging on the Gulf of Aden. They did strike Afghanistan in this way in 1998, when the Clinton administration targeted Osama. The Tomahawks flew out of the Arabian sea, through Pakistani airspace, and into Afghanistan. If the US so wishes, and if they are determined enough, and are willing to bear the pollical cost, they could force us to accommodate them once more. It's a highly improbably scenario, but it may follow if Kabul looks to fall.
To that end, every external power is trying to keep the Afghan government going, keep the Taliban from sweeping too easily. It's even within our interests to have the Taliban impeded a little bit so that we aren't blamed by everyone for allowing a Taliban regime to seize Kabul.